Effects loop problem...high pitched whistle

[ftwizard]

I'm getting a rather bizarre problem when I instal an Alesis Midiverb into the effects loop of my recently renovated Studio .22 combo.
I'm getting a high pitched whistle and the signal becomes a bit garbled, almost like the input is being massively overloaded, although this is not the case.
I've tried swapping V2 & V3, but it made no difference. It does the same on either channel, and tweaking knobs make no difference either.
Any ideas what the problem could be?
Cheers.

 

[stokes]

I dont use or work on/with effects,but I do know some effect units just dont work well in some effect loops,they do "overload" them,made to be used in front of the amp.Try the Midiverb in front of the amp and see what happens.