Regarding the issue of the center speaker being set to +7.5, assuming the left and right are also SVS Primes, it would help if we see see a photo of how it is placed relative to the TV, the left, right speakers and the main listening position.
That could be just another case where you speakers were getting some room reinforcement that got leveled off by Audyssey; and you are one of those who actually prefer some bass bumps at certain frequencies to flatter and/or smoother bass response. As Pogre mentioned, feel free to increase the level for the subwoofer by a few dB and see if you like it better.