Well, I heard back to my last email, and BAT claim that my problem was a thorny one for which they cannot provide a simple solution. After changing out parts one by one, they ended up replacing them by the handful. The bill comes to $300 and I am told the channel is clean. The problem of course is that since I sent the amp to them the problem seemed to result from my PS audio preamp instead, so I wonder if BAT have replaced parts without need. But presumably, they checked first to make sure that the channel WAS making noise before replacing even the first part so I guess I have a new channel with no noise. That the unit has been out of my system now for more than 22 days means I have made a record, but as yet their site does not reflect this.
Am waiting for the channel to get back to me so I can reinstall it and check out the sound. In the interim, I'm grooving to the updated Wyred MC4 fed by my old McCormack TLC1, using both the passive and buffered outputs. Sounds pretty sweet but the Wyred needs a few hundred hours more to break-in.