Endish, I've been hesitating in replying until I could think of something sensible, but decided to reply anyway. As Dr.Horse suggested, temporarily set the sub trim on the receiver all the way up to see if that "wakes up" the sub amp.
One puzzling thing is the failure with the CD player direct to the sub. That would seem to indicate that the amp was shot, but you say that some LFE was experienced from the sub during movies, so the amplifier has some life, but seems to require an unusually high voltage input(higher than the CD player could provide)to turn on.
