Thank you all for taking the time to respond and to offer such helpful advice. Although I understand a bit about frequencies and room modes and non/omnidirectional subs, I couldn't help but wonder if having four subs, including two *behind* me, would change the presentation of the full range of music, perhaps making it more "immersive" or opening the soundstage. Everything that you're saying about a smoother bass response does make complete sense. I'm just going to have to try it and see (or hear)!
going from 2 subs > 4 subs
I'm hoping that someone can explain the acoustic change, the change in the character of the music, in going from two subs to four. I have a relatively small room, 13'x20', with a low ceiling that slopes on the sides with the roof. I currently have two small REL subs in the corners of the front wall. It sounds great and it's made me curious about adding two more, most likely in the back of the room behind my listening position. I've read about the advantages of "the swarm" and other four-sub setups, but most of the comments have been about smoothing out the bass. I'm curious what will happen more generally to the character of the music itself in the room. Will it sound more "holographic" or will it fill the room more? Will it change the soundstage? Will I even notice it? Or is it all too room-dependent or system-dependent to make any definitive guesses?