PB-12+ It will embarras your 505, IMO. I have a PB-10 in a much larger room than that and it does an excellent job already. I wanted a bit more rumble than the PB-10 has, but for the price and size, it's pretty amazing. I have heard the PB-12 in a slightly larger room, and it has a bit more rumble down low, but more or less similar output in it's higher range. I've also heard the 25-31PCi in an even larger room, and it seemed to have more output than the PB-10 to me, about that of the PB-12, but SVS said I'd need to move up to the 20-39PC Plus or PB-12/2 to get a substantial gain over the PB-10.
Hehe, better check the dimensions, because the PB-12 is already pretty good size, but they get much better WAF in the nice finishes.
Perfectly square rooms can be problematic for good bass too. Are you looking for ridiculous amounts of bass or just "a lot"?