Okay play a pink noise signal to your sub and adjust the gain until you reach 75 db. Hopefully, you do have an SPL meter. Once you do that, then run Audyssey. Afterwards, I go back in a set everything to "Small" with a crossover set to 80 Hz. YOU want to check that the levels that Audyssey set and it should be no less than -3.5 for the sub. If it did, then there is a problem somewhere. When I initially ran Audyssey, it set mine to -12 db. But, I did not level match first before running Audyssey. Try that and see what happens. YOU can always bump the levels up a tad, but I would NOT recommend doing it too much. Be careful on that one.
Cheers,
Phil