Unfortunately, this is one of the only pieces I bought second hand off of ebay. The guy is sending me his original receipt to use for any warranty work needed. It was purchased according to him from Good Guys in CA.
Here's the bad news first...Klipsch warranties are not transferrable. From the Klipsch warranty: "This Warranty is only valid for the original purchaser and will automatically terminate prior to expiration if this product is sold or otherwise transferred to another party. "
So far I've pin pointed it to be from the KA.
You need to verify this by sending it a signal from another source. I doubt you have a signal tone generator laying about...so try this. Turn off your receiver. Turn off the power to the KA. Disconnect all sources from your receiver. Disconnect the sub cable from the receiver sub out, and connect it to the center pre-out. Make sure the again KA is turned off, then turn on the receiver and make sure its volume is turned WAY down. Now, power up the KA. Any hum at the very low volume? If not, slowly turn up the volume to see if you hear the hum. If there is no hum, you have a problem in your receiver's sub-out, not the KA. If you still have the hum, try the same procedure with another receiver of any kind, sub-out or not. If the hum is there with another receiver, it is probably the KA.
What's the good news? Maybe you will find the source of your problem is not the KA.