I recommend that you tread carefully with buying a sub for the XPLs.
Maybe you can borrow one to see what it does for you.

I always felt sure I was missing something when I read the XPLs specs, down 3db at 55 hertz I believe. So I scrounged up a crippled Velodyne 18 series 2. The amp/accelerometer was screwed so I hooked it up to a crossover and a 600 watt Crown amp. Add three XPLs as LRC and I was smugly content that I had the best HT setup in my hick county.

One day I was listening to some kind of crash, bang, thump movie and smirking with pride at the low frequence output. I glanced at the equipment rack to check the meters and saw that I hadn't even turned on the sub amp.

Now I'm beginning to wonder if I even know what good bass response is.
Yeah, the Velodyne holds it's own in movies with galaxies exploding but most of the time it's presence isn't even noticeable. (as it should be, but I had dreamed of something more.)