Yet another hum question (yes, I know - and I DID search the forums first)

I have a persistent hum problem from the main speakers.  First, I'd like to describe the system, then, I'd like to tell you what I've tried so far:

Dramatis Personae:

Samsung 65" TV (2 conductor cord)
Ethernet Switch (wall-wart transformer)
Cable box (2 conductor cord)
Oppo BDP-105 (grounded cord)
Yaqin SD-CD3 tube buffer (grounded cord)
Emotiva XPA-5 amplifier (grounded cord)
two PSA XV-15 subs (grounded cords)
Surge suppressor (power source for all but the PSA subs)

But note that ALL these items are grounded to each other via signal-cord grounds (via Ethernet, HDMI, or audio-signal cable grounds).  Also

What I've tried so far:

Unplugged the Oppo - still hums
Plugged the Yaqin into an isolation transformer with the Oppo unplugged - still hums
Plugged the Yaqin and the amp into the same isolation transformer (with the Oppo unplugged) - still hums
Plugged the TV into a (second) isolation transformer - still hums
Plugged the ethernet switch & the TV into a (second) isolation transformer - still hums
Unplugged the subs - still hums
Turned off the Yaqin - still hums
Unplugged the Yaqin - NO HUM

Something in the Yaqin's grounding is causing the hum (my conclusion - do you agree?).

How do I isolate the Yaqin's chassis from the power amp to stop this hum?  The signal grounds seem to be the (unavoidable) connection between the amp & the rest of the system, but isolating the Yaqin's chassis via an isolation transformer did nothing to stop the hum.

Could (should) I crop the grounding plug from the Yaqin's power wire?  That would essentially leave it as a floating ground but for the signal-wire grounding connections to the Oppo & the power amp.  Or, to ask the question another way, is it credible that the Yaqin's chassis is sufficiently above ground potential that I would damage other equipment if I removed the third ground pin of the Yaqin's power cord?

Thanks - Boomzilla