Are the speakers you're using shielded and how close are they to the monitor Skamp?
Bugger, Skamp, that sounds like a problem. Do you think, maybe, the two wires across the rear of the screen are prone to vibration?
Never heard of that one before.
Perhaps get a square of very dense foam rubber, the type used for lining flight cases, and cut a square to go under your monitor base.
When I used to run a disco, one particular rig I was using with two 15" bass bins each side, I used to suffer bad feedback playing vinyl, couldn't crank the bass up that much without suffering the problem.
Got me's two oblong sheets of that stuff and put 'em under the bass bins - no feedback The foam absorbed the vibrations getting fed back to the turntables.
This foam stuff is very firm, it doesn't feel like it would make much difference, but it does.
Try looking in your yellow pages for a local supplier then see if they got an offcut. They'll probably give you a bit for nothing.
I know it's a lot of pissing about, but belive me, it could work.