Well, the problem seems to have resolved itself!
I just had my monoblock amplifiers returned to me, modified last week.
They seem to have broken in during my long listening session yesterday, with the result that the whole system opened up, and lo and behold, the channels equalized.
I know I mentioned the problem occurred with other amps, but it wasn’t as acute. Anyway, for now, I’m a happy camper!
Thanks for all your suggestions.