Anyone gone from a single REL sub to stereo subs?

Phasing is more critical with subs. Both channels will tend to have similar bass information, because the original music will have been largely non-directional, i.e., present in both channels (reflected sound in the venue/studio). If your subs are separated by a distance equal to a half (or one and a half, etc) wavelength, they could be out of phase and their waves could cancel. One solution is to stack them. (Of course, this may not be feasible with the RELs because they fire downward and require a floor.) Drawbacks with stacking include loss of stereo imaging capability and magnet interactions if the magnets are unshielded. I have stacked subs and use VPI Magic Bricks between the subs to help shield the magnets from each other.

I have a friend who upgraded his system and now owns two (2) REL Stadium II subs. He was not sure what result a second sub would bring. Well, his system improved with the second sub and you can easily hear and feel the difference. These subs worked well with Merlin full range speakers. About six months ago, he changed to horn speakers and regardless, the REL subs worked equally well with both set of speakers.

I highly recommend going with stereo subs. My experience are with the REL Stadium IIIs. I started with a single and added the second sub within 2 months. As best as I can describe it, the sound of the system "relaxed" with the added sub. It's a subtle difference, but one that I find well worth the expense. Integrating the second sub wasn't twice as hard as a single sub, but it's at least 1.5x.

My room is 31' by 14.5' and the main speakers are M-L Odyessey.