Something isn't right. If it was me, I'd do an REW measure of my receiver only to check the crossover.
To do this you first do an REW measure of a loopback cable to be sure it returns a flat response. Then include the receiver in that loop, feeding the soundcard line-out to the receivers AUX-in and then the receivers SUB-out to the soundcards line-in. You have to play with the levels a bit, but it's easy to do.
Now the measure had better track that 80Hz target in REW.....
brucek