"like old people pissin cuz it all depends" your system is only as good as your weakest link...what are your subs and amp, is your box custom builtto the specs of the sub or is it just some general box, how are your rcas and power/grounding wires, will your aftermarket H/U have any form of preamp
maybe im just an audiophile because i put everything together myself, and everything DOES matter.
but generally speaking, of course an aftermarket H/U will do you wonders, and always remember you get what you pay for...so dont skimp with some $40 pioneer atcircuit city because that eclipse cd7000 will blow it out of the water, plus it makes the girls moist when they see that you can change its colors lulz.