Please help Sub not working

I have had a large sub hooked up to my HK Signature series 2.0 pre-amp for a few years. I wanted to get a smaller sub becuase I was buying some new furniture. My old sub (JBL) always emitted a very small humm and a couple of weeks ago, I unplugged the sub cable and the outlet and realized it was coming from the power strip; I simply plugged the sub into a different strip and the hum was gone. I thought nothing of it--however the sub stopped working! My new Earthquake sub came in the other day and I hooked it up with a new sub woofer cable..and it doesn't work either.
I've doubled check the settings on the pre-amp and they look fine; When I test the speakers in the set-up mode..they all work except the sub. I've tripled check the cables and everything seems fine. With the AVIA audio test, all speakers work except the sub.
What have I done and what should I do???? Did I screw up my pre-amp?
thanks in advance