Originally Posted by mannoiaj
I think the 2nd sub would give you more than adding rear surrounds or a 3 channel amplifier. After you have the 2nd sub in place, then down the road you could throw up some room treatments, rear surrounds, and an additional amplifier for the front stage in that order. That upgrade path would seem to make very good sense in order of importance and impact. That's if you decide you need additional amplification at all to begin with. Once you treat your room, it'll be much easier to determine whether or not you'd need amplification imo.
This is my vote as well, with the caveat that you get Omnimic or Rew to optimize the sub placements across your listening positions. Dual subs will enable you to get much better frequency response proper placed, but they could actually make things worse if not done right.
It's amazing how tighter, deeper bass can impact the entire sound of your system. The improvement will of course depend on your room, but I've seen properly placed and measured dual subs improve SQ so much it was like there was a speaker upgrade. Certainly a much greater improvement than I've ever seen from the addition of an amp.
Just my 2 cents.