I agree with @noble100 that the bass improvement when going from one sub to two is on the order of twice as good. I stopped at two for now and I am very happy with what I hear but I know the folks with four properly set-up subs mostly report even further improvements. If you could wait a bit until you can expand your budget the (award winning) swarm system by Audio Kinesis ($3,200 for all) seems bargain priced for what you get and may actually be less expensive than piecing something together and then upgrading.
Which is better for a DBA (Swarm); powered subs or unpowered?
I want to start building a swarm (starting with 2 subs), on a budget. Starting with $1000, am I better off buying two used powered subs, three less expensive used powered subs, or a subwoofer amp (eg Dayton SA1000) and two (less expensive) used unpowered subs? What is the advantage of having a discrete subwoofer amp? Room size is 13'x22'.