Subwoofer insight.

For me it's the extra bass "range" that my subs provide as my main speakers have very balanced bass to their obvious limits (about 58hz)...the subs just make it all more believable. 

The SVS 16s are much better than the RELs for half the money.

rwwear,
"The SVS 16s are much better than the RELs for half the money."
There are no absolutes in Audio, only preferences. I've owned the SVS 15 and SVS 16. I currently own REL Carbon Limited subs. I would never own SVS subs for 2-channel again. 

I have a large room too.  It's not as large as yours, but larger than many.  Anyway, I have a pair of T/9i and Revel F208 mains.  I got a T/9i, but in the large room, it wasn't enough.  So, I changed to one R528 (moved from the HT room).  The R528 was good, but I really felt that 2 subs would be better.  I decided to put the R528 back in the HT, added a second T/9i to the stereo setup, and now I have spent some time with it.

My advice is to use the high level inputs as described in the REL manuals.  I have the mains plugged into the 4 ohm tap on my Primaluna HP Integrated, and the RELs are connected as stereo (see REL setup instructions) using the 8 ohm taps.  This is the way REL recommends, if you read up on their installation instructions from their web site, or in the manual.

Once it was all connected, I spent some time tuning everything.  Then listened.  And tuned some more.  And listened.  After doing this for a few days with a variety of music, I have it dialed in.  At first, I thought it would be better to crossover at a low freq, but then I found that it worked better if I raised the crossover, which is what the manual recommends.  I also found that it was better with the volume levels set higher than I expected.

Overall, the sound has improved for all frequencies.  Everything smoothed out, and the system sounds better than ever.  The subs energize the room, but are not boomy, and they don't overpower the main speakers.

IMO, the T/7i may not have enough power for your room.  But, then again, I don't think you need to jump to major subs, like 18 or 21 inches, as some suggested... unless you want hip-hop bass. 
  

Bottom line, watch the REL videos on how to tune, and follow what's on there web site.  You'll adjust them differently than you expect, and the SQ will definitely satisfy!