I think that having it on a different circuit increases the chances of having different ground potentials creating a current loop and hum. It all depends on your wiring. Even a different outlet on the same circuit can cause differing ground potentials.
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Twb2, if I understand you right, you're suggesting that I make myself a long extension cord? I can hard-wire a new outlet close to where the sub will sit with about 2 hours of work chiseling a space in the brick to place the box and then fishing the wire down to the basement. This would definitely be my preferred method of handling it, because it's safer and more aesthetically pleasing (just in case one day I have to consider the WAF).
Bob Reynolds, have you experienced the hum when they weren't on different circuits? I'm just trying to get a sense of the probability that I'll have to deal with this.
Sufentanil, sorry no I don't. My main system plugs into a single outlet via a SurgeX SA1810; my office system is into a single outlet as well. I also use a Jensen isolation transformer on the cable TV feed. If it's not a big deal to retrofit another outlet near the sub, then why not wait and see if you actually have a problem. You could use a Jensen subwoofer isolation transformer to break the ground loop if it occurs.
Jensen Subwoofer Isolation Transformer They will build one with XLRs if you need.