1 large subwoofer or 2 smaller ones...?

As they say in all things audio, it depends. I have two. The only real way to find out is buy two identical units at the same time with full return privilege, that means no restocking fees. Try to get a 60 day return privilege as you will find it takes awhile to find the answer. If you buy only one, you will never know if you are missing something, and that will drive you crazy, as all things audio can do.

2 smaller sub(s) if your room can support. I would strongly advise to demo REL prior to any purchase. These are the best subwoofer (s) on the market. They do not disappoint! Keep us posted. Happy Listening.

I have purchased a few books on audio room acoustics (Sound Reproduction by F. Toole and Master Handbook of Acoustics by Everest and Pohlmann) and both books suggest to have 2 subs in a stereo and HT setup to minimize standing waves. Will also give a break to your main speakers having less bass to handle :-)

But two. One sub always sucks. I use two JL e112 and have tried one for a bit and integration was very bad with one and much better with two.

When is comes to sub bass management, more subs is always better to properly load the room and to reduce modal effects. If it's is worth doing, then it is worth doing right.