REL505 vs SVS 2xSB13 or 1 x PB13

My home theater system consists of Integra DTR9.9 integrated, PSB synchrony 1 fronts, PSB B6 surrounds and a REL R505 sub. My room is about 20'x30' with vaulted ceilings and opens to the kitchen. I use the system for blu ray/dvd music video's, regular CD's and cable, pretty much in that order.

I'm thinking of upgrading my REL to either 2 SB13's or 1 PB13 from SVS. I'm pretty certain that the 2 PB13's will outperform the REL, but will they outperform the PB13? Which would be more musical? On the SVS website the specs indicate that the PB13 digs deeper into the lower octave than the SB13, which may be nice for the deepest organ notes. In this price range are there any other subs that I should consider?

Thanks.