[Clive197]

I considered this avenue and ruled it out after test of an "IFI Audio DC Blocker - 10A" filter : presence of hum.
Also, from my internet research, it appears that the 4B3 has a DC blocker at the power input which effectively eliminates the problem. Afterwards, maybe I'm wrong?

It may not be the power amp that’s the issue, it could well be higher up the chain. Pre, Source, which is why I used the block that covers my entire system.

[Speedskater]

Power line DC offset is seldom 24 by 7 in homes.
But it is easy to check for, by taking the amplifier to a home in a different neighborhood.

While some legacy or boutique amps have this type of problem with high AC line voltage, I don't think that a Bryston amp ever would.

[Rusty Jefferson]

Power line DC offset is seldom 24 by 7 in homes.
But it is easy to check for, by taking the amplifier to a home in a different neighborhood......

In post #19 Fab77 says he tried it in 3 different houses in 3 different cities, and all 3 had the transformer buzz.

[Fab77]

This is not possible, the first 4B3 purchased and the second one received as a replacement are buzzing with nothing connected except the mains cable (no speaker cables or modulation cables).
This has been checked in three different places and cities (among other things, to eliminate any problems with my electrical installation).

It may not be the power amp that’s the issue, it could well be higher up the chain.