Re: what to do to my car?
I'm guessing he's keeping the 500 wrms amp he's got, considering he didn't mention a change in that.
However, I would take the $400 you have available, and get ONE 15 and a cheap amp that'll put out some decent power. I'd say run a BRX1100.1 on a SA15 and call it a day. I'm sure it'll sound way better and be much louder than the P3s or whatever you've got now. But you'd also wanna spend another $60-70 and buy the materials to build a proper ported enclosure.
Head Unit - Kenwood DDX419
Front Stage - Kicker ZX200.2 to Infinity CS60.9
Substage - Sundown SAZ1500D @ 1 Ohm to SSA XCON 15
Enough Knu 0 Gauge to go around