Levi,
I have played around with the PLII music center setting because at a default setting of 3, my center seemed to be the only speaker playing. I'm not sure if I adjusted the value down to 0 or to 7, but one or the other adds more of the center channel into the front left and rear, giving (in my setup) a more even wall of sound. My center is not a good match for my left and right fronts so 3 might be a good value for people with better matched speakers. It should be obvious from trying both extremes which adds more of the center to the left and rights.
I actually like PLII music, but I find the rears are distracting. When I get the matching IB1 center, I'm going to play with the rears turned off in PLII music.
Page 29 of the SP-1.7 on Bryston's web site has a pretty good description of the DTS center value. Page 28 describes the PLII music settings but it isn't as detailed.