Thanks for the replies all, super helpful! So I sat there for 5 minutes turning the crossover, sensitivity, bass and freq pots back and forward, then fired up the amp again. Same thing

So I thought I'd open it up and take a look. Its in pretty damn good shape! Only two caps had any kind of 'oxidisation' around them, and it was a very tiny amount. SO I thought I'd plug in a 300.2 I have. SAME ISSUE! So I followed your advice stipud (from Amplifier Distortion and DC-Offset Info), and tested the outputs with my DMM I have no idea if its relevant for car audio, but I tested all three amps I have (2 chan running sub, 4 chan running mid/high, and 2 chan (the problem child) running mid bass. Here are the figures:
SIDE LEFT RIGHT
SUB 0.1 0.1
Midbass 4.3 1.1
Midrange 7.4 11.4
High 24.2 1.8
So according to this, my mid/high should be distorting more than my mid-bass. but they're not!