Thanks for the advice - I could try my noise suppressor, but it seems like a fundamental thing.
Yeah if I have the drive down on the screamer its fine, but turn it up a bit and it squeals.
I've tried my mates BOSS GT8 into my amp for a laugh once, and found it almost unusable, because even with only delay (no drive or anything), you have to have the settings turned way down or the amp just goes nuts squealing/feedback. Well it's not really feedback...
I've used my tubescreamer into a 5150 before and it worked fine, no 'squealing' noise.
Just wondering if anyone else has mesas that are tempremental with pedals. Cheers!