For a two-channel system, you might consider the new REL subs, which are a bit smaller and offer better performance than the ST series. I had a Stadium III for a few years and bought a B1 a few months ago. I haven't heard everything, but I think the REL subs offer great build quality, integrate well and are competitively priced. I am in no way affiliated with REL or Sumiko.
http://www.sumikoaudio.net/rel/idx_b.htm
http://www.sumikoaudio.net/rel/idx_b.htm