Another update as this thread has seems to have died 🤣
R21 is now 1.1k and C15 2.2uf, made a nice difference to the drive channel and the tone controls seem a lot more responsive.
So list of mods I've done are:
Master volume (R39)
R15 now 1.5k
R21 now 1.1k
C15 now 2.2uf
C3, C5 and C6 removed
Standby switch deleted
Fan power moved from valve supply area to before the 5 diode's (V1 - V5)
That'll do I reckon, very happy how it sounds now.