When playing heavy palm muting on the lower strings, the amp plays at normal voume, but as soon as I play anything else, the amp goes really quiet. If I let a chord ring out, it starts off quiet then suddenly jumps back up to normal volume for a couple of seconds until I strum again, solos just sound like they are being run through a compressor.

Ive put 3 different guitars (both active+passive) through 2 different leads through the amp, and the same problem occurs.

Any ideas whats up with it?