I wonder if anyone can help!?!

I have just had an old Marshall tranny amp come into my possession - it is the transistor version of the JCM800 (5210) and is meant to roar!  However, despite the clean channel having all the correct Marshall tones, the boost channel is thin and very quiet and appears to have some clean mixed into it - louder than the rather fizzy sounding distortion in the background...  I have cleaned the pots and switched jacks but no change...  I also took out all four op amps and replaced with sockets and new op amps of the correct type - that also made no change...  What would be the next move?