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
What do you mean by not working? No sound? Won't turn on? You did mention that you pluged the sub into a different power strip. Is it possible that the power strip is bad? Did you check the on/off switch on the strip? Some strips has fuse or breaker built in. Check the fuse and push the reset button if there is one.
The Sig 2 does not output to the sub in stereo mode unless you get the 2.12 update.
I'm sorry. What I meant to say is that the sub output doesn't work if the front speakers are set to large unless you get the 2.12 update.
You've got to track problems like this down by process of elimation.

Feed the subwoofer output from the pre-pro to the channel of your amp that you have connected to one of your mains. Do this using the the regular interconnect from your mains, not the one that you would normally use to connect to the sub.

Obviously, this means that one of your mains would be temporarily disabled, but this would allow you to see if the pre-pro is actually outputing a signal from the subwoofer output. If it is, you should hear the deep bass coming out of the one main that you selected. If you get nothing out of the main, even after fiddling with the pertinent subwoofer output level controls and adjustments, the pre-pro has a problem with it.

If you hook things back up normally and you still get nothing out of your subwoofer, your cable going from pre-pro to subwoofer is bad. It is possible that you had a poor connection causing the hum and jostling things around a bit gave you a better connection temporarily. This would have negated the poor connection / hum until the connection finally broke, resulting in no signal to the sub. Sean

I rechecked everything as advised and RWWEEAR was correct--the problem lies with the HK pre-amp; the front speakers had to be set to small for the sub to work. By the way, Rwwear--do you know where I can get the upgrade??
thanks to all that responded
Bob Popham
RJPC Design Consulting
360 Rodell Dr.
Grand Junction, CO 81503
PH 970-245-4393
FX 970-245-4396