Got the back off after a good pry/tug - it was lightly glued on, as well as the six screws. Did not expect that...
Anyway, after removing, applying contact cleaner, and re-installing each tube, I cranked it up, and voila! No cutouts, nice loud sound all the way up!
By the way, yesterday I cranked the amp in the "downstairs music room" a.k.a. the pool table room beside the pub. That room has carpets, drapes, and a big blanket/weave hanging on a wall - very neutral to music. Today, after the tube re-fit, I plugged the amp in the nearest useful outlet to my work area (kitchen peninsula), and the hard surfaces of that environment were as huge a difference as you might expect. It surprised me it was so drastic, and I know what to expect. So if your guitar/amp sounds a bit off, try a different room environment!
Using the manual to figure out how to remove the back and each tube really helped with...oh wait, yeah, PRS doesn't do manuals to any great extent. Not even online versions. That is one thing I find a bit bizarre. Although one with some basic info was provided with my Archon.
Anyway, here's a couple of shots after being moved upstairs to the music room, with the Archon to the right:
And yes, I did a few back and forth comparisons, and they are indeed very different animals, though part of that may be due to the 100W vs 30W rating (I should repeat the experiment with the Archon in half-power mode at 50W). Still, the EL34s vs the 6L6s are another obvious factor. And, well, everything else about these amps!