If you are running the system as I understand it- a pair of 8-Ohm top-boxes on one side, a sub (8 Ohms?) on the other- you should be OK.
My concern with this is the power amp. You made the statement "3/4 power"....how do you figure that? And a good, strong transient like a screamer or a pounding kick drum can easily clip an amp even with a dbx driverack in front of it. There are too many variables in live sound to say that the amp won't clip because of a certain setting. So just be careful, don't let the driverack mislead you to a state of complacency. Watch the amp! And good luck.

You don't say which model of Klipsch speakers you are using, but I can't believe they have just a 15" bass unit and no midrange horn or tweeter for the higher frequencies. So you have to take care about the power versus frequency you may be driving them with, especially if your power amp goes into clipping. Clipping generates high frequency harmonics at high power, and it is these that can blow tweeters. Counterintuitively, it is often better to use an amplifier that is overrated for the loudspeakers and, if necessary, perform hard limiting at the amplifier input in order to limit the generated harmonics and hence protect the speakers, never mind the ears of the poor listeners.

I assume you already have electronic crossovers ahead of your amp inputs, so you could consider using a separate power amp for the sub and drive the two main speakers from the two main power amp channels. If you only ever have a mono source, you may even get better results by running the main amp channels in bridge mode, if you can wire your mixer to put out phase and anti-phase outputs, and driving the two main speakers in series. The cabling losses are reduced, allowing more of your amp's power to reach the speakers. If you happen to have the 900A amplifier, that model already has built-in bridge mode operation and could deliver over 1KW to the two speakers in parallel (4 Ohm), but you need really thick speaker cables to do justice to this.

well, the style of music we play is roots reggae. We don't have any screaming, but we do have a lot of keys that are out front in the mix and put a lot of emphasis on the bass. By 3/4 power I was referring to the amp channel levels. The power amp available is an AB Precedent 900, and we also have a separate Precedent Series 600. What I think you guys are saying is to bridge the power from the 900 watt amp and send it to the 15" tops in series to get the best results. How much power should i get to the sub? if it is rated for 400 (i believe @ 8 ohms) then would 300 watts from one side of the AB 600 not be enough power? We were hopping to use at least one side of the 600 for our monitors. If we were to purchase one new power amp for this system, what should we get? We are running mono.

OK, a number of points.

The position of the level controls on the front of the amplifier has little to do with the amount of power the amplifier can deliver at any one setting. They are there to match the amplifier inputs to the output levels of whatever is driving them (your mixer in this case). Some amplifiers have additional power limit controls, but I don't think the Precedents have them.

As for standard vs bridge configuration and whether you run your speakers in series or parallel depends on several things. To run a stereo amp in bridge you have to drive the two channels with exact antiphase signals and then connect the load across the two hot outputs, leaving the cold outputs unconnected. Not all amplifiers behave well under these conditions, so it would be a matter of experiment. Connecting speakers in series needs double the voltage (which is what bridge mode gives you), but it also reduces the damping factor as seen by individual speakers, which can make bass difficult to control at high powers. Having a separate sub woofer gets round this problem but only if your crossover cuts the bass to the main speakers.

I would not try too many changes at once. I suggest as a first step that you try running the two main speakers conventionally from the two outputs of the 900 amp with the channel inputs paralleled. Whether the 300W from one side of your 600 amp is enough for the sub depends on the efficiency of the sub and the acoustics of the venue, but it's worth a try. You would then use the other side of the 600 for your monitors as you suggest, but they may not give you enough on-stage power.

If you are looking for another amp, go for one that gives you flexibility by being able to be run bridged using internal connections only. A 500+500 or 600+600 is a pretty standard unit these days, with a wide choice.

I think what I am going to do is set up using the 900 to power the mains conventionally as you recomended, and try the 600 for the sub and monitors. I may end up buying a new power amp with a little more features, mainly limit controls and an internal bridge mode as recomended, and use the 900 for the sub and monitors. The 600 might not provide enough power to run our 4 stage monitors and sub, but I will give it a shot. Thanks again for all your help.

No, the 600 will not be enough to handle (4) floor wedges and a sub. Especially for a reggae band, where strong bass is a major player in your sound. You need the power in the low end, where bass "eats" power in ANY system. This is why you see all of these bi-amped "active" speakers on the market that have roughly twice the power going to the LF speaker than to the HF components.
In addition, putting (4) wedges on a single channel of a power amp can be a load that is less than stable. A typical wedge represents an 8-Ohm load. This is not a hard-and-fast rule, but a majority of them are just that. This means that when you string 4 of them together in a typical parrallel manner onto a single amp channel you are placing a 2-Ohm load on that channel. This can be very taxing on the amp, generating too much heat and lowering its damping factor to where the control over the speaker cones' travel is not "tight". I will say that if your wedges are rated at 16 Ohms, this will yeild a 4-Ohm load, not an issue.When you add a SUB to the other channel and pounding that with LF program.... you will have another issue-crosstalk. There is that potential to have the LF info from the driverack's LF out going to 1 channel "leak over" into the monitor channel. This can create additional problems, mainly due to the fact that wedges weren't designed for that kind of bass material. Another failure in waiting.
I would get a dedicated power amp for the subs. Until you do, you can try putting the (1) sub on one side of the 900, put the (2) tops on the other channel. Then use the 600 to drive the wedges, putting (2) of the (4) on each of that amps channels. This way you are ensuring that there is no load less than 4 Ohms on any channel, you will have enough "oomph" for the sub, and enough headroom on the FOH to keep from killing a driver.
This also has the benefit of being able to create a pair of wedge mixes as opposed to just one (assuming your mixer has at least 2 Pre-fade mixes, most do these days). Even when you are running a "mono house" a band can benefit greatly from (2) stage mixes...
A note to Boswell: I don't know if the dbx Driverack is available to the UK market (I thought that it was). This is an active processor that incorporates electronic crossovers, EQ, and dynamics control (including peak limiters) in one unit. The power amps are fed by this unit, not the mixer. The mixer feeds the Driverack.

ok, so what you are saying is that if we can get the volume we need, it would be fine to have the two tops on one channel of the 900 and the sub on the other. The only concern I have is dammaging the speakers due to underpowering them. We currently have it set up this way, and it seems to work fine. How would I be able to tell if the power amp is clipping? there are no level meters on the amp, all we can see are the levels on our mixer, and ofcourse use our ears. So far, from what we have heard, everything is clear and nothing clips. Is it a bad idea to have the amp channels turned to the max and the mixer to 3/4, basically maxing out the system? Since the dbx is between the mixer and the amp, it limits the mixer output so that it does not clip (and we wouldn't let it clip at the mixer). If there isn't a problem with giving two mains rated at 500 each (8ohms) set up in a series 450 watts (4ohms), and 450 watts to a sub ratted at 400 (8ohms?), then this would really help us out because we would have everything we need for the system. The idea of having two monitor mixes on stage is great. Thanks again for all of the feedback, I am gaining quite a bit of usefull knowledge through this forum.

OK. For starters, people put too much in the power rating of the speakers, IMHO. That rating is to make sure that the operator doesn't overpower them, and the manufacturer is basically covering their butt on this, IMHO.

As for the clip indication, I am looking at the AB website. The models you listed are in the "Archive" section (I guess that means that you bought these used?), but the PDF spec sheet states that they DO have clip LEDs for each channel. That's better than nothing...SO.
First, you need a test source to generate a continuous signal to calibrate with. Your Driverack provides a "pink noise generator" to set up the system from the DR on through to the amps. BUT, if you record a good, clean CD of the DR's output while it is delivering this pink noise, you will now have a source to run through the mixer to help calibrate the system.
Simply take a portable CD player (you know, the one you'll be using to play break music from on the gigs), and play that pink noise CD through the line input on a channel (Note:NEVER run a source like a CD player thru the XLR mic ins on a mixer; if the phantom power is on, you may easily damage the player. Same goes with wireless receivers!). Anyway, with the CD of the pink noise, and the CD player, you are armed with a decent test source taht will help calibrate the levels. Play the CD through a channel, setting the Gain control on the channel so that it isn't lighting the clip LED. Bring up the faders so that you are getting a nice, strong reading (typically "zero"-the last green LED) on the mixers' outputs to the dbx. Then set the inputs on the dbx per the op manual. Stop the CD at this point, set up the dbx per the manual, and then proceed to use the dbx's pink noise generator to feed the power amps (whose level controls are DOWN at this point!). With the outputs on the dbx reading at their "+12dB" point (last green, which is 10dB before the dbx's clip), slowly bring up the power amp levels. Pink noise is LOUD, you'll probably run everybody in the room out! Set the power amp levels so that they are ALMOST igniting the clip LED. IF the dbx manual tells you to do something else, DO THAT! But his should be good, unless you let the dbx go into the "red". I'm tired just thinking about this.....be sure to follow the manuals' instructions on how to set the Peakstop limiters, EQ, etc.
Another tip: your dbx has 2 channels of processing. Use one for the mono house mix, use the other on your "frontline" wedge mix, and the other wedge mix can be for the keys/drummer/backline. That way, you have all the benefits of the dbx, including feedback control, on the mixes that really need it...

I never thought of using the other channel of the dbx for our front line monitors, that is a great idea. I will follow your instructions and use the pink noise to calibrate the system so I will know that I am not clipping. I feel much more comfortable about our system now, and I understand it more. Thanks guys for all your help, I have learned so much valuable info from this forum.

First, you can't do that with the dbx Driverack PA. It is not discrete stereo, or dual mono. It is stereo processed from in to out. The dbx 260 is a true stereo processor, allowing you to run dual mono sources.

Second, A/B's are not very strong amps. Since I don't know exactly what model you have, I am guessing, and unless A/B did something weird one year, you cannot effectively run mains on one side, subs on the other, because the channels share one power supply. You can only do this well with amps that have discrete power supplies. It may work for a while, and it may work at low volume. But I will put money on that amp tanking on you while the toking is getting good.

I have been thinking about replacing this amp for a while, it is the source of all my problems, as I'm sure you know by reading this topic. It is old, and I aquired it through my family so I did not invest money in it. What are some good power amp models I should look at? As of right now, we are using an older Sunn Professional Series mixer (mono) so we have no need to run stereo mains. I am thinking that something that puts out around 1200 watts would be sufficient for our two mains (set up in series). Are there any downsides to bridging an amplifier's channels to get the maximum output as a signal signal?

I DID ask what model Driverack was being used, as that detail was not stated (unlike the model of the AB amps which were stated as being the 900 and the 600). I said that if it was the 260, he could do exactly what Sheet said it could do: dual-mono. Sheet is correct in that the model Driverack PA is strictly a stereo unit. We were not told which model and, frankly, my personal experience is with the 260, that's what I went with.
But you know what happens when one a-s-s-u-m-e-s. And, yes, there are many amps on the market that are better performers than AB. QSC, Yamaha, Peavey/Crest, and some Crowns (Sheet has said the cheaper models aren't very good, but my experiences with Crown have been solid) all come to mind.

I didn't read the model A/B model number, sorry. Usually when I see A/B, I have bad flashbacks, smell burning transformers, etc.

As for cheeper Crowns, you should talk to an honest tech at a factory authorized repair station. See if they can send you in the right direction. It isn't the budget models that are always the issue. It is the topology and manufacturing.

Crown had a boat load of contractor series amps, the C2000s, that were manufactured with mismarked components. So, they had major failures. All amps were replaced no questions asked. Thumbs up there. But the fact is that the weekend warrior may be out some time and hence money waiting on a replacement to arrive.

It can happen with any manufacturer. It just so happens that since many of the key players that made Crown amps great have since been shaken from the tree, that there are new minds, new technologies that aren't as solid.

Consider the I-Techs. Great idea, huge failure rate.

I guess I need to do some in-depth research. The one thing I am sure of is that I want plenty of power for this system. As moonbaby stated this style of music calls for powerful bass, so I want to make sure that the sub gets enough power. Also we have a lot of high-mid range instruments (keys and guitar) and of course vocals so we need ample power to the mains as well. I will be heading to Anaheim next week for the NAMM convention so I suppose it would be a good opportunity to check out and new amplifier technologies and talk to the companies in person. It would be an investment for me. How long do power amps normally last with moderate use, and with a clean style of music such as reggae?

Well, Sheet, I have to admit, the Crowns I am most familiar with are the classic (i.e.,old) D/DC series (150, 300, 75), and I've still got a pair of Micro-Techs that are running. As are the D150 and the D75 in the studio.
I currently use (4) CE2000s and (4) CE4000s, and with the exception of a screwed-up connection that caused a channel to DC and take out the sub it was driving, I've been pretty happy with them. They work well with my Crest 7200s, which, alas, are no longer available.
Tim: Be sure to visit the QSC booth when you go to NAAM.

Good amps will outlive you.