HMV 2304 Racord player/Radio combo.

[Murphyv310]

The manual is easy to find as per Dave's instructions Ref. The amp has NFB and the PSU is a simple half wave type. Amps like this can be prone to AC pickup and its known that leakage from the on off switch to volume control if combined can give hum at anything other than minimum volume. Id remove the mains lead connections from the switch if combined and hard wire them temporarily away from the switch and see if the hum goes. I'd also get the hum issue resolved and then see if the discrepancy in bias has changed.
Also check the VC is earthed properly and there is no hum loops, the triode is very high impedance so very easy to induce pick up.