OK, got a chance to play a bit on the weekend. Here's what I found with PLIIX Music:
Panorama - sounded neat (more hall like) but not as natural as I would like (varied with source material), so decided to leave it off
Dimension - I had the same observation as you Chris - seems to move the soundstage forward or backwards - I ended up leaving it at default as well.
Center Width - found a setting of 2 or 3 works in my room to blend the center with the L&R and not have the vocals shackled to the center channel alone.
Once these tweaks were made and similar tweaks to Neo6 Music, I found there was little to decide between the 2 modes. I now like PLIIX Music much better than I previously did.
On a side note - I realised I must remember to turn my center back down to its true calibration setting for music as it's normally bumped up a few dB for movies.
Cheers,
