I don't know for a fact, but would running the center +4 at the channel level introduce extra distortion?

My first test would be turning the center down to 0 and turning everything else down to negatives accordingly.

second thought, mains are at 0. Has this been propoerly calibrated? Mains0 Centre+4, surrounds+1 sounds like a by-ear setup.

snazzed

M22, VP150, QS8 <--all v2
Sub: Outlaw LFM1-Plus
Denon AVR1910, Sony X900-65"