the bass stand sw520 should be available very soon, uk importer is waiting on delivery,
the advantage of the bass stand over an active sub is that they roll off the bass from the s-520 below 200Hz , this removes all the strain put on the little mid/bass unit which should give better power handling and cleaner mid range performance, the bass stand has a quoted 38HZ low point but i reckon that this is forward output from the bass driver and will most likely get down to 30ish Hz in room , you could always run an active sub too but it will have to be a goody to have a LF upper roll off at 25Hz 24db octave to stand a chance of integrating with the sw520.
cant wait to get my hands on them.
regards