In the 30+ years(ouch!)that I've used a variety of Sunn tube amps(primarily for bass)I've always noticed a point of diminishing returns at about "7" on the volume. Anything past that does not increase volume, but creates that special Sunn-specific distortion I nick-named "the burn". Not creamy Marshall break-up, not compressed bright Fender surf-tone, the Sunn burn reminds me of the buzz you'd hear standing next to a giant electric power transformer. The conventional wisdom on this is that 6550 or KT88 power tubes provide very clean power up to the very end of being pushed(therefore being well-suited for bass amplification), and then finally break up with a very nasty, brittle overdrive. Sorta like transistor amps, huh? While this may be true, I think the interaction of the preamp tube driving into the power amp tubes has something to do with the saturation. So, assuming that all is well with your amps' capacitors, etc., you might want to experiment with using a preamp tube with slightly less gain, like a 12az7, kinda hard to find, but it will give you alot more headroom, especially if you have a bass with active pick-ups. Also, just for an experiment, if you can run your head through a very efficient modern speaker cab for contrast, do so. Also, remember that introducing the low-boost switch will tax the power tubes a bit, as you are introducing a tonal filter, just like down-shifting a car to go up a hill will take stepping on the gas pedal.
