Turntable/Subwoofer Ground loop

You didn't say what turntable and cart you are using.

If per chance you have a DD turntable, and are using a moving iron cartridge, (such as a Grado), this is most likely your problem.  The moving iron cart picks up emi from the DD motor.

Make sure your subwoofer is secured properly to the floor. I had serious resonance feedback/hum when I put the REL subwoofer on a Subdude Auralex platform.  It took me some time to figure out it was the platform -  then ended up blue tacking the subwoofer to my wood floor.  Got rid of the resonance and now it works fine. 

The problem may also be that your turntable is not isolated properly.  What kind of rack do you have it on?

Mac, you say the "hum" begins as soon as your stylus hits the record which means it is your turntable. The Grado problem was already mentioned but other cartridges can be afflicted by a poorly shielded motor. As you move the cartridge closer to the motor the hum typically worsens. If your tonearm is too light for the cartridge or the cartridge too stiff for the tonearm You can excite a resonance which will feedback through the subwoofer. Adding head shell weights will stop it. How much depends on the mismatch. Get a Hi HI News test record and add weight till you get below 12 Hz but no lower than 8 Hz. I like between 8 and 10 Hz. I think you get better bass (more detailed) this way. Also if the subs have room control if the level is boosted at the right frequency and the turntable is in a "peek" location It will make things worse. 

Good Luck,
Mike

To get rid of EMI you need a turntable mat like Micro Seiki, SAEC etc  

If you are not using a 3 wire speakon cable, make sure not to use two negatives on the high-level input. From speakers: Red and black on left speaker. Red on right.