Just to be clear, do you mean the transformer hum is back?
A ground loop if the hum is from the speakers, but not if it's transformer hum.
Yes...I am not talking about speaker hum.
This is transformer hum so the hum is from the amplifier transformer itself (not speaker).
I ordered the proposed solution (CMX-2) to DC offset: https://www.pooraudiophile.com/2015/03/how-to-fix-dc-offset-and-transformer.html
CMX-2 has two outlets , so initially I used one for the AMP (with transformer hum) and the other one connect to power strip for pre-amp, players, etc). That did not eliminate the transformer hum.
Only when I connect just the AMP to CMX-2 , leave the other outlet alone/unused , then the AMP transformer hum is gone.