this is a bit odd to me..... Bats, the SVS ultra-13 in your room may not look as good as you want, but I have never heard such a bass response in any room. If you don't like the "look" of it then maybe you should design a cover of some sort and make it look like part of the cave. There are many transparent materials that you can hide that beast in your room with.
Honestly man.... That thing works perfectly in your room and gives a complete "pressurized" feel to the sub. It's awesome!!!! It bested (2) Dynaudio 500 subs!!!!!
My advise, don't bother with an IB sub. Just hide the SVS with some transparent get up. Don't try and fix what's NOT broken. You already tried an upgrade to the dual Dynaudios.... and that was a failure. Keep the SVS..... If you want to get crazy then buy a second SVS and make them BOTH hidden into and part of the cave.