Its also odd, to me, that two very different products have the same LED arrangement and trigger on the same phenomenon.....
Odd indeed. But if a high amplitude subsonic oscillation is coursing through both components, per the phenomenon described in my previous post, it would seem consistent with ...
b) one of the components affecting the other.
The hypothesis described in my previous post is also consistent with the fact that the problem didn’t occur when a different amp (and a Bryston amp, no less) was used with the Bryston preamp, that amp having a far lower (50K) input impedance than the ARC amp with which the problem occurred and therefore being much less likely to trigger the phenomenon I referred to.
Also, another factor pointing toward power supply instability in the 12B as being the root cause of the problem would seem to be the fact that it is 25+ years old.
Regards,
-- Al