According to "Dennis" at the Bryston Service Office in VT,
my amp has been repaired and is being shipped back to me. (I have confirmed the fact that the amp indeed went to UPS today and I have a tracking number.)
Culprit: the "Series/Parallel" switch was defective.
If the "guaranteed" delivery comes to pass (Friday, 18 June), that's not too shabby a turnaround: 11 days, including 8 days transit (ground)* up and back, Fedex/UPS respectively, and involving a weekend of no activity.
* Interestingly, the shipping rate for guaranteed 3-day service is over twice the ground rate, which is also guaranteed 3-day service in this particular set of circumstances. I KNOW I'm in the boonies, and can only speculate from perusing the map that the Bryston facility is similarly geographically challenged; this may explain the disparity. (Remember Clooney's line from "O Brother....": "You're two weeks from everywhere!")
My only complaint is that I had to call them to get a status report, which seems to be SOP these days.
As a retiree I easily forget the fact that they (Bryston) probably have other kettles of fish to tend in addition to those I inflict upon them, however.
Hopefully I'll be able to put my 27 year old Optonica SA5151 receiver back into the closet. I've gotta say, however, it's capable of pretty decent sound, when paired with my Burmester 001 (feeding directly into the main amp section of the receiver) and Maggie 3.5Rs.