It was really odd, and for now, it's gone

; all I hear now is a very very faint hint of 60 Hz which is only audible with my ear right up against the loudspeaker in both channels. This is with a cheater plug in place, and it's much quieter than anything which might annoy me. Previously, the cheater plug only made a small difference, and the noise was more of a buzz (60 Hz plus harmonics I guess). It would remain quiet forever until moderate-level or louder music was played, and then the buzz would appear and remain forever until I turned the amp off. In other words, there was no refractory period, and the buzz would never appear unless I played moderately-loud music first. Weird.
Since I got the new tubes, I set the bias as described in the manual (starting low and checking after 20 min.). Yesterday, the buzz in the left channel was there. Then, heard some slight popping in the speaker at startup and shutdown in the left channel. I decided to set the bias at .280 instead of .300 (mostly because that's one of the only variables I can manipulate), and I've been "light loading" since I replaced the tubes. Today, no buzz, vanishingly low noise, and as always incredibly good sound. However, I *think* that the problem disappeared even before I tried under-biasing, so I'm not sure if that made any difference. It seems like after that initial popping, the problem is gone.
Admittedly, in my search to find the culprit with an intermittent problem I could be connecting things together in my mind that aren't actually connected. If so, the coincidences sure are weird.
To be sure, I'm really happy with this amp despite what seems like some idiosyncratic behavior. Hopefully that's over with.