If you've had it serviced by Rita, and it is functioning well otherwise, then I would say the hum probably IS a ground loop issue, and inserting a grould loop isolator probably IS a proper solution. I have an older Velodyne sub (F-1500b) and it works wonderfully, UNLESS it's plugged into the same outlet (or power strip) as my Cable box, or ANYTHING that is connected to my cable (like th receiver or TV). Cable TV cable systems are NOTORIOUS for having wonky grounds, and sometimes they can cause very difficult to cure ground loop issues. The only way I got rid of the hum in my old setup was to 1) connect the cable box to the receiver with a TOSLINK cable (so there was no electrical connection, only fiber optic cable), AND 2) connect the sub to the receiver with a GLI box inserted between the cable and the sub, AND 3) to put the sub on it's own power strip, on a completely separate outlet (which was on a different breaker).
In my current system, the cable system doesn't seem to induce hum, but I've still got the sub on a different outlet (because of where the sub is located, relative to the rest of the components). But now, I've got a completely different problem with the Velodyne. My interconnect between the sub and the reciever is about 4 meters long, and now, any time my sweety and I are sitting on the couch, and have our cell phones sitting near the sub (both are "smart phones"--a 3G iPhone and a 2nd gen BlackJack) and one of our phones receives a call, about 5 seconds before the phone rings, the sub will "chirp" with a high-frequency digital "twitter". It's so quiet that if we're watching a movie with the 5.1 system playing, we don't hear it, but if the receiver is off, and we're just watching TV using the built-in speakers, this digital phone "twitter" coming from the sub is REALLY annoying. It took me about a week to figure which speaker the noise was coming from, and now that I know, I STILL haven't figured out how to remedy THAT little issue...
Anyway, it has been my experience that about 95% of the "hum" issues with a subwoofer in a multi-channel surround rig are due to ground loop issues, and about 50% of those issues are DIRECTLY attributable to the wonky grounding protocols of the cable system...
Ground-loop hums are a HUGE pain in the butt, and can be REALLY tricky to track down and kill. GL hum is like a ghost--it can come and go, and just when you think you've got it fixed, it can come back at the oddest moment, without reason or warning.
Good luck figuring this out, and welcome to the Carver Forum!
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