right now i have a pretty crappy old system, its a sony xplod mono channel amp @ 4ohms..and 2 10" kicker CVR's ..both are no longer made

i also have a kenwood excelon kdc-x493 head unit..which i like alot.

my budget is around $700..and im unsure of what subs and amp to get, im leaning towards possibly a 15" JL w3.. dunno what amp to get , or if that would even be the right sub for me. i want something with tight bass that hits alot harder than my 10"s now..
so what is a good sub(s) for me to get ,and a good amp?
i have a 2006 subaru impreza 2.5 btw.