I haven’t used either of the amps in question but from experience I know pedals can sound very different going into different amps, so I’m not suprised it sounds different going from a solid state modelling amp to a small valve head.
I also think fuzz pedals usually sound much ‘fuzzier’ or ‘fizzier’ going into a solid state amp, I think valve amps tend to smooth them off slightly.
but saying all that, what you are describing sounds more dramatic than that…. although I’d be suprised if something is “is going on” with the Big Muff, more likely it just isn’t suited to the amp… unless it needs a new battery or something of course?
