Love the soundstream amps, definitely a great budget amp. I'm running a full size 4-ch Picasso and a 2ch tarantula and have no complaints. Check out ebay for better pricing though. As waffles said, don't underestimate an 8" sub in a proper enclosure. I'm running an infinity 8 in a homebrew cubby box, it's amazing how much bass can come from such a small sub. Also as others said it probably wouldn't be a bad idea to run some type of EQ to help tune the system in.

 

Oh trust me, I would never underestimate a 8" sub, a buddy of mine runs eight 8" subs in his build, and that thing HITS !

 

Clarion makes a neat little 5 channel amp I've been watching for some time under $200. OP, I feel you on the no head unit dilemma. One less reason to break in the vehicle.