Spin had a lot of good tests, I would also check settings in headunit to make sure "sub out" is set to on.
I would double check the online fuse, actually, just switch it with a spare, it may be possible that the power light you see is just power from the remote turn on wire. A longshot, but worth double checking.
Did you actually try putting a multi meter on the amps output(while music is playing)? That would narrow down if the power is getting all the way through the amp and the fault is with the sub.
If there is no power coming out of the amp, then check if there is anything coming out of the sub preouts of the deck. That should narrow down the issue.