Subwoofers hum - Vandersteen 2wq - can't stop it

I have two Vandersteen 2wq subs. Each of them hums through the drivers (not from the amplifier plate). The hum does not sound like 60 cycle hum... it is more like a buzz maybe in the hundreds of hertz range.
The buzz is audible even from 6-8 feet away. It is constant and happens regardless of whether the speaker cables are connected to the subs or not. So with the subs plugged in and no other connections made, they hum. With everything connected, they hum the same way. Also, each sub is plugged into different AC lines... one into the same line as the other outlets in the room (including dimmer switches), and the other into my dedicated 20 amp AC line with the rest of my audio system. I can unplug one sub or the other and it has no effect on the one still plugged in.
I won't bother giving details of the rest of my system since it seems not to matter. Is something wrong with my AC?
Have you considered trying Audioquest subwoofer cables? Each has a 3rd ground wire you can connect to a screw on the back of an amp or AVR for example on one end and and a screw on the back of your subwoofers on the other. This really does work well and it's eliminated the hum I was experiencing. I have two subs as well in my system. Another suggestion is to replace your contractor grade cheap electrical wall outlets with PS Audio power ports. I've done this and it does make a difference.
I'd suggest you try contacting Richard Vandersteen. Richard does not suffer fools lightly (no insult intended) and he can be abrupt, so first make doubly sure that they are connected as described in the manual. I assume you know that they require their own special crossover inserted between the pre-amp and the power amp and that they are driven off the main amplifier speaker outputs. This is unique in the industry and is said to contribute to their ability to blend well w a variety of amplifiers and speakers.
is your source grounded 3 prong plug
hook up a test ground wire from the Sonos chassis to your pre amp chassis Call me at audio connection
My apologies for never responding. I expected to see emails if anyone responded to my post and so I never checked back! If anyone is still out there:

Swampwalker: Yes I have the subs connected correctly and I use Vandersteen's best crossovers, the M5-HP. I have not called Richard but if, after I move to a new house in one month it does not alleviate the problem, I will.

PDdn and Audioconnection: I have not tried the Audioquest, but I did run a ground wire from the subs' grounds to the chassis of my preamplifier. No help.

Again, they hum even when completely disconnected from my system, so that seems to rule out ground loop hum?