Hey Bert - you ever figure out what was wrong with your amp? I'm looking at picking one up for way cheap; the seller describes it as having a problem that sounds just like what you're saying.
Peavey Unity Koa, Ibanez RD500, Squier VM 70's Jazz, Fretless P-Mutt, Fretted P-Mutt, Squier Jazz Deluxe Active V
SWR Fan Club #140: Henry the 8x8, Super Redhead
GK MB500, Neo 112-II