Thought I should also report some of my experiences with my current mods.
So far I have jumped the 3 100 ohm resistors on the cathodes, switched the gain pots on channel two and three to 500k and 1M, and as TheMagicEight suggested, I added 5M5 resistor in parallel with the 680k resistor (well, practically 4,7M resistor in series with 820k resistor).
I like the changes a lot. Especially for leads. Overall, the amplifier is tighter, has more urgent attack and I think the growl has more mids in it.
Channel 2 with 500k pot on vintage mode has a vocal like quality to it and simply sings. It does tight metal rhythm very well on both vintage and modern. It still handles lower gain stuff decently.
Channel 3 with 1M pot truely has tons of gain and tightness. With rest of the mods, there's no need for boost at all. On the other hand, I can see why many people think 1M pot is too much as it pretty much kills the option of dialing lower gain tones which don't sound thin.
I'd even go further to try the 250k presence pot , but then I'd need to find a stacked pot with 250k/25k values as this one is a Multiwatt model.
After all this I'm still somewhat confused whether pre 500 rectos had 250k or 1M pots. There's conflicting information about it all over the place. The rev c schematic in this thread has 250k pots, then there was that another thread where someone heard from Mesa that all the recto revisions use 250k gain pots. Yet, I've seen comments on some pre-500 Youtube video where the uploader told he had gain set at 11 o'clock or so and the sound was pretty much on high gain territory. So could someone clarify what's the actual gain pot value in rev c/d? If someone actually bothered to measure the gain pot on rev c/d, the case could be finally closed
