Well, this thread convinced me - I've gone and bought a TH350, should be here in a week or so. All the real-life stories from gigs and practices that talked about how potent this "little" amp was sold me on it. In the end, it just didn't seem to make sense to pay $200 more for the few extra watts of the TH500 that probably add up to about 1.5dB of extra volume (i.e. just perceptible), especially when that $200 gets me a good part of the way towards adding another cabinet and increasing my volume that way. It was also rather persuasive to discover that the power module in the TH350 is the same one that was in my Shuttle 6.0, and I found that amp to be plenty loud for all the applications I used it in.
I cannot wait for it to arrive, and to give it a good run-through!