Hi,
these suggestions WILL change the sound of the amplifier and may also upset it's operation, but probably worth trying. Be prepared to revert to the original values.
Set the two input filter time constants to about 80mS to 100mS and to 0.75uS to 0.3uS.
This could be done retaining the 220pF and the series resistor reduced to 1k5 to 3k. Alternatively parallel the 220pF with a 330pF polypropylene and use ~1k0 as the series resistor. This works best with a low source resistance (Rs) unit preceeding the amplifier, certainly <=300r and preferably ~50r.
The bass/DC block could use the original 20k and a new 4.7uF film cap.
If the source also has a DC blocking capacitor then the pair of capacitors work in series and the usual formula has to be applied to the combination to then calculate the time constant.
The NFB DC blocking time constant does not need altering to match these input modifications. It can remain as 1k0 & 220uF giving 220mS. If you wanted to experiment with a better capacitor here, then do not drop it (or a parallel combination) below 150uF.
