KEF Audio PSW 2150 Subwoofer Speaker Subwoofers

I bought this sub used for $20 to pair with my kef q15s.

This sub has a issue with ground hum.I don't think any body including kef has figured out what causes it.Ive taken sub to two authorized Kef repair centers which did not help with hum. I also switched receivers which helped alot but still sometimes it...