Michael, one of the main benefits of 2 subs, is inspite of what you may have read or been told, there is some amount of directionality to a sub. Two subs allows F/L F/R placement to obviate this effect. Also, for lesser subs (read less $$), each sub has to work less.
I find that with the ASW4000 in our family room, due to its huge woofer and amp, this is not an issue at all. And in fact, started with one ASW3000 in the dedicated HT room, and within a day ordered the second. Hope this helps,