Your two PA cabinets wired in parallel will get you 850 Watts (4ohm) from one side of your amp. Careful with the guitar cab. If it is a 4 ohm cab you will be giving it almost twice its rated power handling capability.

Mike Miller, post: 371844 wrote: On channel one I have a 475 watt guitar cab but its in mono. Next I want to run two (Behringer PA speakers) on one channel..the second channel. How would I go about this and what wattage /ohms would the speakers run at if both ran on one channel.

First off, why do want to run a guitar cab on one channel? You say it's a 475W cab. Program watts, peak watts? What? Which cab, with what speakers? If it's an 8 ohm cab, it'll get about 500W, possibly. (8Ω / both channels driven / 1 kHz: 500W). 4 ohm, possibly 700W (4Ω / both channels driven / 1 kHz; 700W).

Now, for the Behringer 'Sure-Fail' speakers. 8 ohms each. As mentioned, paralleled would be 4 ohms. (Amp out, channel two to first speaker, out the paralleled jack of speaker (?) to other speaker). Amp power about 700W (4Ω / both channels driven / 1 kHz; 700 W).
Behringer Power Rating: 800 Watts Peak Power; 200 Watts Continuous. I'd be extremely cautious of believing any dubious claims of a speaker being able to handle peaks of 4x the power rating of 'continuous'. Honest companies generally list it right about double, as in 200W continuous, 400W peak. (Look it up). (Cheezy companies using questionable specs to sell their inferior-quality cheap products do that kind of misleading advertising).

Anyway, so figure a (questionable) 200W continuous, 400W peak from those Behr 'Sure-Fails'. They'll have about 700W available to split between them, (4Ω / both channels driven / 1 kHz; 700W), so it would be about 350W per speaker. That's doable. Gives them a bit of headroom, if you don't go crazy pushing too much through the amp, with it at full volume on that channel.

Again...why do you want to connect a guitar cab to the amp?

Kapt.Krunch

I'm running an all rack mount digital setup now so I have nothing to power it. I figured that was what you do (This is the first time I've ever used or needed a power amp), but anyway the cab is 8 ohm's and the tag on the back says eminence speakers and 475 watts (Guessing thats peak) it has a main and an ext jack and Im running it in mono. And what speakers do you suggest I get? I dont need top of the line stuff It's just band practice we just use the gigs stuff for when we play live. Anything has to be better then my 200watt each powered harbingers...

I suppose if you already have all that stuff, then figuring out how best to use it is the right thing to do.
if you ever want an alternative... I have been succesfully running a full band practice room on one QSC K10.
I never would have believed that would work had i not tried it. Loud enough, and clean enough.
price-wise ; I think K10 is well comparable to the price of an amp plus a bunch of speakers.

my own $0.02