Klipsch is known for making very efficient speakers not subwoofers. Yes i a gree, the PB-12 Plus should be quite a bit better than the RSW-15. The PB-12Plus is an excellent sub that is a top performer in its class. There is no doubt in my mind on it being one of the best picks.
Dont worry about matching brands. A good subwoofer is an accurate subwoofer. You dont have tonget a Klipsch sub because you have Klipsch speakers, in fact, i would prefer not to match them. Most here would agree on choosing your speakers based on personal listening preferences & choosing a sub based on how well it measures. SVS is well known for its excellent subs, driver quality, enclosure quality & most importantly how well their subs perform!!
In a large room id be looking at either the PB-12 Plus or the VtF-15H. My choice would be based on how much money i can spend on it. The VTf-15H is the best bass for the money. The PB-12 Plus will beat it, by $500...? I cant answer that but it is a bit better.