[bikerhifinut]

I spoke too soon.
The amp seems to be working just fine and dandy and sounding like it should.
The issue is that the "slope" filter now seems inoperative.
Its described as an adjustable low pass filter working above 6kHz, variable from zero to 20dB per octave above this frequency. it looks like a complicated treble cut network inserted between the first 2 gain stages. What I can't understand is if I had made a mistake, it would be unlikely to affect both channels equally and as it's in the signal path with 2 capacitors that I replaced then errors there would result in no sound.
I shall have to look at it but without any actual service data all I can do is check voltages around the active devices.