Here's what i'd suggest:
in the clipping path, there are 7 caps.
Try bypassing each cap with another cap of the same size. Remeber that capacitors in parallel add...
If this gets you further away from the sound you want, you probably want to decrease the size of the initial cap.
Fatter distortion means you prolbably wanna increase the pre-clipping treble roll off slightlly, and maybe increase the gain a little.
Taming the shrill high's will mean you probably want to increase the treble bleed off across the clippers.
You could also tweak the distortion texture by using different diodes, I'd suggest that if you do, use 2 in series on each, ie: 2x1n4148 || 2x1n4148.
All the usual diode tricks should apply.
I'd focus my attention on the 330pf caps, the 1.8k/10uf and the 1.8k/0.1uF parts, making thorough use of my audio probe to work out what the circuit was doing, since i can't visualise what was happening.
I'm guessing(can someone with a more electronic trainig confirm) that the 2 back to back fets form an inverter. If that is the case, the 330pf's across the inverted/noninverted signals would cancel some treble and the 1.8k/10uF would bleed some of the low frequency content of the inverted path to ground, when combined via gain pot, this would introduce a low frequecy boost...
Changing the caps will change the locations of the rolloff, chaning the 1.8k resistor will change how much of a low freq boost you get.
