Apples to oranges.
If the sub doesn't work when you feed a line level input from the receiver, but it DOES work when you feed it a speaker level signal, then you still have the possibility of the sub's amp being blpwn.
Try feeding it from another line level source. Any line level source., say a CD player
If it stills sounds silent, then, in all probability, it's the sub's amp that died.
It may sound bassy and bad, but it it sounds at all, you've proven that the sub's amp is working and you've isolated it to the receiver.
I'd start checking all my settings there.