There are alot of good subs out there. And if I were you I would do some looking. Audiobahn makes some hellacious subs, so does Diamond. Memphis, Kove, Oz, also. Money would be a factor to. For some of the best sound, try Bass Box. It's a company called Harris Tech that made that up.
For pure power, I would go the commercial amp way. Like Carvin, Sun, QSV. When you buy an amp like MTX, RF you never get out what they say you would. But if you buy a 2000 Watt Carvin, you actually get 2000W of pure sub power. And you would have to buy a DC to AC convertor. And the convertor better be one of the best.
Also, if your doing 4 subs, I would either do two 15s, and two 12s, or two 12s, and two 10s.
If your doing 5 subs, I would do two 12s, two 10s, and one 8.
6 subs, I would do two 12s, two 10s, and two 8s.
That way you get low bass, mid bass, and high punchy bass.