Strange buzz issue!

If you used a ground isolator, the PS ground should fix the problem. It is the voltage offset between the two PS on the two pieces. What is common without a common ground, the audio circuit ground. You may have ran a wire but it was not to the right place, I'm thinkin..

A ground isolator in the signal path is the fundamentally correct way to eliminate system ground loop problems. ... For an unbalanced interface, the transformer effectively stops any current flow in the cable caused by ground voltage differences, which stops the noise.

I would look to the VAC ground between the two, just by moving alligator clips to different locations.  Might surprise you.
I'm also thinking eliminate the ground on one unit (via cheater plug). Then run the ground wire between the two and only use the ground on the other unit.. I expect that will do it... A little cap trickle.. Common single wire ground should do the trick..

If all else fails a GL eliminator, but it can KILL the dynamics of some systems.. Sometimes the signal won't activate a Sub when there is a GL eliminator in the circuit.. Dayton plate amps.. I know what your talking about, "impaired bass response". I have to use one on a cable box power supply.. It is the actual CC fix.. 

Just an old mechanic thinkin' out loud..

Regards

Tanks guys for all the great advice! I’ll go ahead and try all of you suggestions! Tomorrow, I’ll be off to buy some alligator clips:) I’ll keep you posted. The buzz aside, I feel this is a combo with great potential, especially  for a fan of a bright, fast and detailed presentation, like myself. If I get it right, I plan to settle with the ARCs for my Yamaha NS1000Ms:) Thanks again!