Back in 2020, there was a good bit of discussion regarding hum when connecting differential output amps (specifically the BHK 250s/300s) to REL subwoofers.
Here is the thread: BHK Pre and Power on the Way - REL Subwoofer Question
Initially, PSAudio provided a free ground lug, to anyone who asked, that could be affixed to the amp rear panel. This did not fully resolve the issue for most - myself included.
REL then offered a resistor network that could be connected between the amp and REL subwoofer. It was offered for the princely sum of $100. Steep to be sure but it resolved the hum issue for virtually everyone (myself included).
