Doc, thanks for the advice! I took the call to start replacing components on the board. First was the pair of 431's - no difference. I then swapped out:
1) 10,000 uf 6.3V capacitor
2) 0.1uf 640v (F104k) capacitor
3) 2 x MJE5731A
4) 2 x 2N2907A transistors
Working on the principle of "in for a penny in for a pound" I figured I would replace all the main components at the same time.
Initial listening makes it sound like the issue may be resolved - I will listen more this evening to confirm but it sounds positive so far!
Quick follow up question - do any of the components I have switched above have a material impact on sound. The capacitor's for example are not an exact visual match (should be an electrical match) - just wondering if you would expect replacements like the above to alter the sound at all and whether I should consider having matching components in the other amp?
Thanks,
Bill