sorry if it seems like i'm chasing you around this board. A feedback destoyer is at it's core a parametric eq. rather than pull a specific frequency if pulls a range (called Q) the middle being pulled the most and tapering out on each side. A feedback destroyer just detects the signals feedback produces, and adjusts the Parametric Eq to kill it. They do have the potential to eat content from the mix you've already created. Possibly getting your stuff sounding tinny. The first thing you should do is look at mic placement in relation to the speakers. Try as much as you can to eliminate feedback before the mix. Then go to EQ. A good method is to use a cd of your group or similar music to set your eq for the venue. Then once the mics are hot, make adjustments for feedback. You do what's called ringing out. I'm sure someone can talk about it, but i can't now because i'm late for something.

jbeutt is pretty much right, especially about trying to get rid of it at the source first. In a bigger live setup you would eq the monitors to cut feedback (placing a graphic EQ on insert for each monitor is pretty common) so that you're not messing up the FOH mix, but in smaller setups this isn't usually possible, or it just doesn't help.

The thing most feedback destroyers have over a standard parametric EQ is a VERY narrow bandwidth, usually lower than most eq's will go. It's more of a notch filter. This will be less noticeable in the mix than a regular eq, but overuse of it will still cause problems.

The pros to feedback destroyers are 1) they're usually automatic, and a good one can see feedback forming before you can hear it 2) They get the job done, and when used right, along with a decent stage setup, affect the mix very little

The cons 1) they can be a bitch to set up, especially if your rig is mobile. Setting it up in soundcheck is easy enough, but you pack the room full of people and you're suddenly in an entirely different acoustic space (although this usually works for the better....usually) 2) I've noticed a lot of them have difficulty with lower frequency feedback.

A band I just did a show for had this little Sabine brick-looking feedback eliminator that just had a few little buttons and one knob. After re-reading the horrid instructions 7 or 8 times, I was able to set it up, and it actually worked pretty well. I just patched it in on an insert. I may buy a few for my little mobile PA. (1604, 2xSRM460, a few 58's)