The X-sls is the floor standing version of the X-ls model.
It puts the woofer in an optimal ported enclosure that will allow it to hit a -3db down point in the mid 40's.
With this version you will not need to purchase speaker stands either.
Another neat feather is that the bottom of the box has a 7" tall area that can be filled with sand to make it very stable.
Oh yea, and they are only $299. a pair.