Do you have any amp techs in your area? I'm sure there are extra gain mods published on the net for the HR amps, or he may already have some; those amps have been around a long time and are plentiful. There are probably mods out there to change the character of the distortion too. Gain can be increased with some fairly simple mods, like resistor value changes in the preamp section; of course with any PC board amp like the HRs, it does take a bit of time to take them apart, change components, and reassemble. I would guess that you could get some extra gain mods done for $50 - $100. The Drive channel has 4 preamp gain stages, you should be able to get some pretty outrageous distortion out of that amp with some simple component changes.
Try the amp section of the Fender forum:
Fender forum