Two Subs v One for a Big Room for 2-Channel Audio?

I have a relatively large room (28'x36'x8') in which I am currently running Acoustic Zen Crescendo speakers with a REL R-528 SE subwoofer (two 12 inch forward and bottom firing woofers). The room is a finished and carpeted walkout basement. The REL sub is placed in the corner of the room, per REL’s set up recommendation, although I’ve experimented with placing it in the center between both speakers. The sound is good with the sub in both positions but, given the size of the room, I’m thinking that two subs—each placed in opposing corners—would give a fuller, more balanced sound. I’ve explored various Audiogon threads and read around in books such as Jim Smith’s Getting Better Sound. However, I am still uncertain if two subs would be best for my room and set up.  Most Agon forums on 2 subs v. one explore the issue for smaller rooms and weigh the benefits of running one bigger, better sub vs. two smaller one. My question concerns running two bigger and (nearly) identical subs in bigger room versus just using one sub.  

My Crescendos sound very good on their own but—again, given the size of the room—the REL adds weight, heft, and realism to audio playback (I’m really not that concerned with video, even though my 2 channel stereo is hooked up to the TV). Locally, there’s a sale for a pair of REL S-5 subs (those replaced the R REL line) and I’m thinking of selling my single R-528 SE and going for the pair if S-5s. The S-5s are also 12’’ dual firing woofers, but the materials used on the R-528 Special Edition are superior. Alternatively, I could go for just another R-528 I’ve found for sale online; however, it’s not an “SE” model so the materials and finish are a little different, even though the specs are the same. Should that concern me?

Any advice would be greatly appreciated about any of these queries, especially the general one about two subs v. one for optimal 2-channel stereo play back.

Thanks very much!

John