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.
