High end sub for big room. Suggestions?

Depending on how you are going to use it and how much you want to spend. If for movies get a 18 in sub the bag end is modest in price and a good peformer. The Velodine 18 is good but is twice the price.

There is no substitute for displacement. The REL may be the right choice because it integrates so well and is musical, but you will not be happy with HT unless you have 2 in your room... or you listen at low levels (U571 was not meant to be listened to at low levels ...) . I am a BIG advocate of stereo subs. Much improved imaging. Further, with 2 subs, MUCH easier to integrate into the room and get it right. I have a dedicated 30x16 room with 15' cathedral ceilings. One sub does not really cut it in term of impact at references levels for HT. Get the REL, get two of them.

2 18 inch Bag End subs with elf stereo processor and I defy any other set-up to top the preformance with either HT or stereo.