You need to consider the entire space for the subs. The subs see the entire open space, not just your listening area.
The question is, do you personally feel your current subs are lacking? While those might not be enough for some people, they might have enough output and extension for your tastes. If you do not drive them very hard they might be sufficient for your needs, in which case there is no need to spend money on other subs.
The RSL Speedwoofer looks fine. I like the Hsu VTF-1 mk3 in that price range, excellent performance for the price, especially in music ranges. Another good one is the Monoprice Monolith 10" THX sub. I would think the SVS PB-1000 is good, although I don't have experience with one myself. These will all be a good step up above your existing subs.
For a bigger step up, look at the Hsu VTF-2 mk5 if you want something that can punch above these 10" woofers class. And the Rythmik LV12r as well, along with the Monoprice Monolith 12 THX Ultra, the Outlaw Audio Ultra-X12, and the ported 2000 series subs from SVS.
If you want to take a giant leap over your existing sub, look at any of the 15" subs from Hsu or Rythmik or Monoprice. I like the 13" sub from Outlaw, and SVS is running a really good deal on their 13 Ultras as well.