In terms of addressing things from the subwoofer side of the equation, I think Josh's battery of measurements is pretty comprehensive. However, the room and poor setup (placement, EQ or lack thereof, etc) can easily spoil your results. From that standpoint, something like the SB13 can have some important advantages over the PB13. Small size is great from an aesthetic standpoint, but more importantly, it opens up placement options that just aren't there for the PB13. In addition, there's the matter of roll off profile which you mentioned. In a smaller room, it's not hard to imagine that a subwoofer with a shallow rolloff starting ~35Hz having a pretty flat response down to 20Hz and below. However, the PB13U in that same room will have a bottom heavy response without correction. Combined, those factors can make a great deal of difference for what you actually get at the seats.