Man the options for a 900$ system only in need of subs and amp is pretty wide ranged...I will give you a couple ideaz to fit the range...would help if you want alot of bass or not??.
Go with 3 infinity kappa perfect 12's with a jbl bp 1200.1 or 2..
you could try 3 alpine type-r 15's and 2 jbl bp 1200.1
or the evermore brahmas, shivas, juggy, idamax,...etc.