I am glad you finally got things all worked out. This is actually what I meant above by but I should have added to set the fronts to small. Sorry!

I agree with Gonk about trying lower crossover settings for the center and surrounds. Listen to some 2-ch source that should have a centered bass guitar with PLIIx, change the center crossover and see if the bass player is centered or appears to move over toward the sub.

Also, you can also adjust the 2-channel subwoofer offset which changes the sub level if there is too much or not enough bass in stereo compared with multichannel.