You can try the powered sub in a MILLION different systems and get hum IF the sub is plugged into a different outlet or on a different breaker.
I'm assuming that you've got the sub at least a small distance away from your source components and as such, it might be plugged into a different breaker / outlet with different ground potential. I ran into this in my house when initially trying to run monoblocks, as i have outlets within the same room that are on different breakers. It was confusing as hell at first, but i was able to track down the problem. The obvious thing to do would be to turn off the breakers that feed the "meat" of your system and then see if the outlet that you have the sub plugged into is still active. If it is, there's your problem. You would either need to plug the sub into the same outlet or at least another outlet on the same breaker.
Keep in mind that two outlets can be on the same breaker, wired with the same polarity and still produce a ground loop or add noise to the system. This would typically stem from having a poor connection somewhere along the line, which could produce a different voltage or ground potential.
If memory serves me right, i also seem to remember that the amps for these are not a "grounded" three pin plug. As such, you might be running into problems with AC polarity. The polarity of the sub could be oriented one way while the source components might be opposite. While you might think that it didn't matter, the whole system is tied together courtesy of the "ground" on your interconnects. As such, this can put a MUCH higher ( actually lethal ) voltage on the "shield" of an interconnect and produce a gob of low level noise / hum.
You can easily check into this with the use of a typical multimeter. If you don't know how, take a look at a thread here on the A-gon forums entitled "Hum, Noise and AC Polarity" ( or something like that ). It's not too old as i posted it within the last few weeks.
The other alternative if all of that checks out good is to pull the sub amp apart and see exactly what is humming. It may be possible to tighten up something, mount the offending "hummer" in a damped fashion, etc... and minimize or at least reduce the noise. Be careful where you're poking around in the amp though, as there might be residual voltage left inside capacitors even if it isn't plugged in.
Other than that, all you can do is to contact the company and see what they have to say. I would not settle for an "oh, they all do that" type of response either. Since they were "kind enough" to take your money as a paying customer, they should be kind enough to offer customer support and stand behind their product. After all, you purchased their product to use in a "high fidelity" system, not a "high fidelity with hum" system. Sean