replaced stock speakers w/ JBL 8621F Concert Series 6"x8" 2-way car speakers

Hey Guys....... Just replaced my stock speakers this past weekend w/ some JBL's. Huge Difference!! All 4 installed took me about 2-2 1/2 hours. Didn't help when I got first door done I forgot to hook back up the handle cable! Eye Roll. Next I will replace the 2 door subs but will need a new amp to run those. Any Suggestions on replacements?? I have seen a couple on Crutchfield ranging from $180 to $300 per sub.