My McIntosh 2105 did that when a power supply capacitor was going bad.
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A batch of 4b-st went out once upon a time with muting switches that were prone to fail over time. When working, the switch mutes the speaker taps at power down, so that you don't have to listen to the power bleed out through the speakers. I had one that went bad, and it sounds very much like what you're describing. Contact Bryston -- yes, they really are fantastic -- and they'll take care of it.