Hi,

Measure the voltage on TP3.

Switch off.

Connect 33K in parallel with R28 and fit 12AU7.

Switch on.

Measure voltage on TP3 and try adjsuting it with VR4 to the same as measured before. If the adjustment range is insufficient you need to change the 33K fitted in parallel to R28. If the voltage remains to low fit a lower value, if it is too high fit a higher value.

Interresting. The capacitor would have added distortion and phaseshift at low frequencies, maybe you need to re-tune the subwoofer?

Ciao T