Keep in mind that after you perform this mod Reverb will not work, so if you are OK with that keep reading.
Here's a picture of the main board and the location of the mods
OP-AMP BYPASS MOD
- Replace C32 with 47n but don't solder both legs
- Lift only one leg of R22 & R23
- Replace C19 with a 220k resistor
Solder a wire from R23's free pin to the 'flying' leg of C32
- Solder a 220k resistor between R22's and R23's free pins
I couldn't find a neater way of doing it but I'm happy with the results. The signal is pretty strong at that point so you can use normal (unshielded) wire but if you want to be extra careful you can use shielded wire. Just remember to ground the shield only once to avoid a ground loop. A good place to do so would be the grounded leg of VR5.
NORMAL CHANNEL MOD
- Jump R1
Remove C4
Replace R5 & R6 with 220k resistors
Change C5 to 47n
Having op-amps in the signal chain forced Vox to reduce the preamp gain in order to keep them from clipping. They solved that using 100k plate resistors on both input triodes. So if you perform the op-amp bypass mod it's a good idea to raise the values of R5 and R6 to squeeze a bit more drive from your preamp into the PI. By removing the treble bleeding cap (C4) you also get the 'original' response out of the Normal Channel volume pot.