In the church sound system, we have a rack mounted DBX sound processor which has an anti-feedback feature. That (allegedly) detects in-phase speaker-to mike feedback and automatically notches it out. Let's just say that it doesn't exactly function to the degree as described. It sort-of works, but a dedicated feedback killer would be a a LOT better.
The other problem is that the DBX is wired only into the main overhead speakers , and not into the stage monitors.
The previous sound guy made a real mish-mash out of the system wiring, and I'm slowly reworking it to a proper and correct configuration. He was a self-described "audio expert", but we are finding that his expertise was mostly just an illusion
Add in a really cheap set of four "Wal Mart quality" 650 MHz wireless mikes, and it makes every sound session a rather exciting adventure
I've seen various rackmount units in the "Pro Audio" section of the Parts Express catalog.
/ed B