Keep in mind that the purpose of multiple subs is generally not more bass, but smooth/equal bass response in various listening positions. One sub will likely set up areas in your room where the bass sounds strong, and areas where it sounds weak... peaks and nulls. Seems kinda strange, but it's true. Two chairs right next to each other... bass sounds strong in one chair, weak in the chair right next to it. Multiple subs, properly placed, will even out those peaks and nulls, so everyone in the room hears the same level of bass.
There are lots of good subs that will provide more than enough chest-thumping bass for the average room. So ask yourself if other people in your house are as critical listening as you. Maybe your wife/kids/roommate are more casual listeners. Maybe spending $1k-2k to make the system sound better to YOU would take priority over making it sound better to THEM. In this case, it may be wise to buy THE BEST single sub you can afford now, and place it where it sounds great to YOU. (It will still sound good to everyone else in the room.) Later, you can buy a second sub, matching or not, to smooth out the bass response for your wife/kids/roommate/friends/visitors/etc and take them from good to great also.