So, I opened it up and started tracing out the circuit, hoping to find some things I could change to improve the headroom of the amp. I actually haven't finished tracing the circuit beyond the first gain stage, but I think I've found part of the reason why the amp sounds so nasty when turned up past 2.
I found a calculator for opamp gain, and if the math is right, with the gain pot cranked, the gain in this circuit is 370. That's way too much gain. Even a vintage single coil will send the op amp into hard clipping against it's meager 14V power rails.
The solution seems simple enough, change the value of R2 to a high enough one to reduce the gain. Problem is, how do I pick a value short of plenty of trial & error? I'd think I'd want the gain low enough that the opamp never clips, regardless of how hot a signal it's fed, and let the clipping diodes make the dirt. But what's a reasonable upper limit voltage wise to design for? Some pickups are pretty damn hot. I'd suck to have to have things that the amp was usless with a neck single coil but just right for a high output bridge humbucker, or vice versa. The other concern is screwing up the rest of the circuit (since I haven't traced it out, and don't know what the rest of it does). For all I know the rest of the amp circuit may need
the first opamp stage running at max voltage all the time to get any output at all.
Am I over thinking things again?