This is for HOME use???
There are TONS or 3 way systems that are affoirdable and include subs. The most talked aboiut onm this forum seems to be the Logitech 2200, although I would never trust them for mixing critical material. Studio monitors need to be free from colorization and a sub almost always adds an unrealistic ammount of bass.
I'm a fan of subs on stage, but not in the studio.