Hey Shawn,
I went from a 16-46CS+ to a PB2-ISD. IMO there is a difference between the ISD and Plus drivers. Personally I like the sound quality of Plus drivers over the ISD's. It's hard for me to put my finger on it, but I would imagine that it's not unlike the diff between the plus and the ultra drivers.
By no means is the PB2-ISD a slouch. You will have no problem filling that space with bass. In fact, the CS+ will probably do the trick, but I understand WAF and all. That's partly the reason I changed subs - it no longer fit the HT relocation.
Something else you may want to consider (depending on your listening habits) is getting dual UFW-10's. With real wood veneers and piano black end caps, it's high on the WAF AND slams some serious bass, but it won't troll as low as the CS+ so you might miss that ultra low bass. If you listen to 50/50 HT/Tunes, you should consider it or give them a demo in your area if possible. I'll be demoing a UFW-10 this weekend. We'll see what happens.
My 2 cents.
