The main thread is here: http://forum.grailtone.com/viewtopic.php?f=24&t=58838
I'd call it more of a "reversion" to old specs than a "mod"
Remove R277 and install a jumper.
Remove R272 and install a jumper, change the cap at C55 from .10uf to .0047uf. (the smaller orange drop cap in front of the larger 3)
Remove R271 and install a jumper.
Remove R110 (680k) and replace with a 470k, remove R322 and C10 (both which are behind that red cap.
Clip and jump LDR 4, just cut the leads as close as you can to the body of the LDR and just solder the leads together on both sides.
You can also just solder a jump across the resistors/caps that I've removed if you want to make this reversible.
Another thing you can do to add a little more brightness/cut is to lift C12, opens up the high end a bit, although this cap defines the Rectifier voice (I like it better with the cap in personally)
This includes changing both gain pots to 1 meg, changing the presence pots to 250k.
Doing this will boost your high gain tones significantly, but will however cause your cleans to have grit to them (but if you slowly raise the gain you'll find a nice sweet spot) It will also clear up that fizz.
I however rarely use cleans so this was right up my alley.
However, use caution while desoldering/soldering in these tight areas, these traces can lift if you use too much heat with your iron.
You can also do this on the tremoverb which is a 2 ch recto, I'd refer to the schematic since they added new parts and moved around others.
This could be applied to the rectoverb/single rec, I'd refer to the schematics.
EDIT: This can also apply to 3 channel/reborn, roadster/road king.
When Mesa switched to the 3 channel heads they used relays for channel switching instead of LDR's so these 100 ohm resistors simulate the presence of the 3 LDR's on the cathodes of the gain stages. You can add a jumper (reversible) or remove and jump these.
You can add 2 10uF / 450V electrolytic caps and put them in parallel with the D and E power rail supply caps. You'll notice a bigger, bolder tone (this is the big difference between 2 and 3 channel models).
Also now find the D power rail. V2 pins 1 and 6 each connect to 100k resistors, and on the other side of those resistors is the D power rail. Somewhere on this rail, you'll need to connect the capacitor (Which is C55 on the 2 ch heads, .0047uf) to ground. I'm not sure where because I haven't worked on a 3-channel amp, but I'd bet there's a spot on this rail that's physically close to a ground.
3 channel pics:
Reborn:
Old 3 channel:
