Okay, the first thing(this isn't silly; it's happened before)is to look at the back of the 3800 very closely and make sure that you have the sub cable plugged in to the sub output in the pre-out section rather than the sub input in the section of multi-channel inputs.
If that isn't it, disconnect one of the mains and connect the speaker wire to the sub(disconnect the coaxial sub cable)as if it was a main speaker and play something that has significant bass in it(turn the frequency control on the sub all the way up)to see if there's sound.
