OK. Here are a few things to try.
Your EP500 Quick Setup guide, says:
If your subwoofer is producing a 60 Hz hum; remove the Chassis Ground Screw. If this does not solve the 60 Hz hum, call 1-866-244-8796 or consult the full manual at www.axiomaudio.com/manual_ep500600.html
There is a diagram in your manual, or it can be found at the web page above, which will show you where the ground loop screw can be found (Step 8). If that doesn't solve the problem, then:
Disconnect the incoming cable line from your cable box. If the hum disappears, you can order a VIDEO ISOLATION TRANSFORMER
from Parts Express that will solve the problem. If disconnecting the cable feed doesn't solve the problem, then:
Try using a 3 to 2 cheater plug, available for under a dollar at you local hardware or Home Depot, on the subwoofer's power cord.
If that solves the problem then try to plug all your equipment into the same circuit. Keep in mind that even if your electrical outlet has two plugs, they could each be on a different circuit. I don't recommend using the cheater plug as a permanent solution (though I've done so many times). Do so at your own risk.
If none of that works, on Monday, pick up your telephone and give the nice folks at Axiom a telephone call. They are exceptionally nice people, and they'll be glad to try to help.