Thank you all for your suggestions.
So, I tried leaving everything connected as-is with the battery and inverter powering the Puritan, into which all components are plugged, as I'd really like to get this to work), but I powered on only the power amp with no ICs from the preamp. Silence (with subwoofer still plugged to wall socket and connected). So this rules out subwoofer and power amp, as well as all tubes in power amp.
I then connected and powered up the preamp and power amp, with nothing connected to the preamp's inputs - the buzzing returned. This rules out the Mac Mini, the BelCanto, and the DAC, as well as differing ground potentials between the components, as they were not connected (at least I think it rules these all out; please correct me if I am mistaken).
I swapped new signal tubes into the preamp - buzzing continues. I tried a new rectifier - buzzing continues. I did not try swapping the voltage regulator tubes, as I can't see why they would be the problem, but I could swap those if someone thinks they could be the issue. The preamp uses 01A tubes which, by definition, have to be from the 1920s or 1930s, because that is when that tube type was popular. I have read they can often be microphonic, but I am not sure if that could the issue and, if so, why swapping through 3 pairs of tubes made no difference.
Does anyone have any other thoughts?
Thanks so much to all of you for your help,
Chris