Are there any manufacturers out there that create a higher gain 12ax7? The average is from what I understand around 100. Are there any that create ones that give 10dB gain on top of that? Or would that make the circuit unstable?

Washburn X50PRO FE (GuitarHeads active bridge pickup)
Dean Vendetta XM
Jet City JCA20H head, w Eminence Texas Heat 12"
Danelectro Transparent Overdrive
Ibanez TS7
DigiTech RP-255
Behringer VD1 (Big Muff PI clone)
Why do you want to do that? 10dB more is a gain of 1000, which your not going to find in most, if not any small signal triode.

Furthermore there are problems with such high gain. You run into a real problem with microphonics. The reason that EF86s have such a notorious reputation of being microphonic tubes is due to the fact that Vox used them at such a high gain everything got picked up.

If your looking for a "heavier" tone then you would get this out of overdriving the preamp more, which can be done by modifying gain stages or adding more gain stages. Swapping tubes will result in some change, but if your looking for that much more gain its not gonna be as simple. If you want more gain then you can try a 12bz7. In the same circuit you tend to get about 2dB more gain per stage with a 12bz7 than a 12ax7. Your not gonna get much more gain than that though.