I had two PB13s and just added a SB13. The difference is staggering. In my large room, it's a totally different kind of bass that really fills a hole I felt I had with the PBs. I have mine at -20 (no time for audyssey yet) and during the one test I did with a scene from IronMan 2, it knocked the magnetic screen off my center channel (at reference). I do feel like I have a localization issue but I'm not goin to worry about that until after audyssey. My advice would depend on how much you love low frequency response. If you want really low, you'll want the PB. Otherwise I'd go for two SBs just for the size/convenience.
BTW, I'm talking about Ultras here. I just saw you said plus.