Like others have done I snipped C3, C5 and C6. At first I liked it but later realised something was missing, you lose treble but you lose dimension too. I fitted a triple pole mini toggle switch and wired the capacitors through the switch so I can switch em back into circuit whenever I need. It's like a bright switch.
I also replaced R3, R8, R15 and R21 with 1K/1 watt resistors. The GZ34 rectifier is now gone and replaced with a solid state recifier, this however has raised the plate voltage to 450 VDC.
I wanted to fit EL34's so I replaced R45 and R46 with 1K/5 watt resistors, popped in a pair of EL34's and it worked fine however with 60mA of cathode current at zero signal I replaced the shared cathode resistor R44 with a 330 ohm 25 watt alternative which is now mounted if front of the fan for better cooling. Cathode current is now 50mA per tube and this amp is running like a dream. The fizz I had with the 6L6 tubes is gone, now I have creamy overdrives and gorgeous cleans.
I use a THD hotplate attenuator so channel 1 cranked up gives the best overdrive tone of all. This amp is superb now. I might yet fit another input jack socket and maybe look into building a tremelo circuit and effects loop. But they are considerations for the future and not exactly easy mods to perform.