I also like the mod3 and the mods systematic suggested back then were so good i kept them in the mod3 boards i sell.Most of the time i make the mods a bit customized to the likings of the customer.The mod 3 was never meant to be a super high gain mod.I also like the clean tube sounds the the mod3 has.This was never possible when the board was heavily modded.It's always a bit of ying and yang i guess.
Now as for plate voltages usually the first preamp tubes have lower voltages,usually in the 150-225V range,the last stage the phase splitter is higher more in the 300V range.This is done to have enough voltage swing to drive the power tubes.This is close to the max voltage specified in the datasheet of the 12ax7/ecc83.I don't think these can handle 400V plate voltage,especially the current tubes in production.