Same issue with me!
Yamaha rx-v375, q acoustics 2020i speakers and svs pb-2000 subwoofer.
On the receiver: subwoofer set to on; all speakers set to small, crossover 80hz; phase normal (but also tried reverse); levels set to zero.
On the sub: Volume set to half, fase 0, crossover set to maximum (LFE).
With the previous settings I can barely hear the sub. If I turn sub volume to maximum and receiver sub level to +10db. The sub plays ok... not loud and not even good enough. I'm testing the sub on a small living room (4m x 3m / 13ft x 10ft) with almost no windows.
Connected the sub to a media player (600X3D) analog out with red and white RCA from the media player to R and L on the sub and the result was great... I am starting to think that there is some issue with the yamaha receivers with some subs....