I own the RF3II series and just recently purchased the 25-31Pci. We listen to music 80% / movies 20%. I spoke with Ron and Tom many times regarding this and my interpretation is:
20-39pci gives you lower extension in bass, but sacrifices upper bass output.
25-31pci gives you the highest upper bass output, but only goes to 25hz.
For my system, which is in a room of 20x20x16 (room actually much larger--this is listening area), the 25-31pci can easily give me 106dbs from my listening position 16ft from the sub. At 25hz, 106dbs, I can feel the bass in the couch and the windows rattle. I do not have my pant legs move however, which is often desirable with SVS bass heads. I was initially told to get the PB2+, I believe, but due to cost and WAF I couldn't. Overall, the 25-31pci is fabulous for music...unbelieveable, and worth every penny. Movies is just a bonus. If you like mostly movies, and like to completely feel the bass, the plus version may be best. But, you will lose some decent bass in the music realm...2-3 dbs, I believe, over the 25-31.
For me, I chose the 25-31 and am very happy. The plus wasn't worth it. However, in the future, I may play with the idea of stereo subs (another reason I went with the non-plus version), and get a second pci, but we'll see.
I think you have to decide of 3dbs of more power at 20hz is worth the $225.