Got a lemon
After a lot of research I decided to buy one of these from Sweetwater. It sounded really good, and was versatile. However, on about the sixth time I used it, it had a failure where the sound level dropped to almost nothing. Then a minute later there was no sound and just a moderately loud hum. I tried different guitars, cables, settings (channel and mode), and power, but no joy. Of course I couldn't recreate the problem, so I returned for a refund - you can't gig with an amp you don't trust. I realize I could have done debugging and wait until the problem happened again (say during a gig) but I didn't buy a nice amp to be a Mesa tester. As an aside, there were two very small issues that (and I might be reading too much into them) worried me a little about their QC: 1) A presence knob was loose, and 2) two of the rubber feet on the bottom of the foot switch came off. Again, small, but for a company that prides itself on detail and quality - not impressed.
Seems I got a lemon, but I'm not ready to buy another Mesa just yet. Too bad - the amp sounded great when it worked... :-/