I agree this is totally bizarre and I've never heard of this before.
Possibly dumb question from a rank amateur: if the Mac amp has such an enormous toroidal transformer, and the amp itself seems to make a low-level noise when on (albeit one the OP has to listen closely to hear), is there any possibility that the amp is emitting a powerful EM field because dirty AC (aka DC in the AC line) is saturating the toroidal and the sheer size of the toroidal is generating a field sufficient to excite those speaker drivers?
If that's at all plausible - which it might not be! - then a DC blocker could test the theory and possibly eliminate the symptoms, even if it would not solve the root cause.