Hey Guys
I will soon be putting my sound system back into my car

I have
2 x 12" Alpine Type-R subwoofers, they are the 4ohm Dual Voice Coil versions
They are being run off this amp, Rockford Fosgate 551s

The subwoofers are currently wired in parallel, and I am going to be using 4awg wiring

Here is my questions for you technical experts!

Question 1: Which Amp?
At some point I am going to change the amp, Would I better better wiring the subs in series and getting a mono block amp? If so, what ratings should I be looking at? Considering both subs are 1500W each, and 500RMS (between them producing 3000w Total, 1000RMS).

Question 2: Enclosure?
After talking to a guy yesterday, He told me that my subwoofers could do with a bigger enclosure? Something to do with airspace? Is this essential? If so, what dimension box should I be looking at getting made up? Should it be ported too? My current box is roughly 15" tall, about 35" wide, and goes back about 20"? rough estimates there, I wil get exact dimensions for you tonight. and it is dual ported.

Thanks in advance for the help,