I dunno... I might not be popular here in this discussion, but oh well here goes: I went for many years without a sub at all, with my Epi 100s... never felt the need. Have one now, though, and do like it, but it wouldn't be the end of the world if I didn't have one at all.
Maybe it'd be worth the effort if I have time, space, money, and the correct tech and meters and whatnot to balance it all out with two subs, but I'll cross that bridge if I even feel like it when the time comes. Maybe if I have a bigger room or something, or just if curiosity gets the better of me. I'm quite satisfied as is, both with my Epi and my Heresy each w one sub in different rooms.
I do like the one dual-side firing sub that I have with my Heresy IV, positioned right between the speakers... and maybe that two-sided firing helps with the "balance" in my room, but I never localize the sub w my ears... it is a complete disappearing act.