Re: 2ohm or 4ohm
2 dual 2 ohm subs will allow a 2 ohm load, while 2 dual 4 ohm subs will allow either 1 or 4 ohm loads (depending on how you wire them). If the amp is 1 ohm stable, dual 4 ohm is the way to go; if the amp is 4 ohm stable, dual 4 ohm is the way to go. If the amp is 2 ohm stable, dual 2 ohm is the way to go.
1 ohm is the most power, then 2 ohm, then 4 ohm.
Headunit: Pioneer Premier DEH-P940MP
Front Speakers: Pioneer Premier 6.5
Rear Speakers: Pioneer Premier 6x9
Amp: Memphis mc1000d
Sub: RE SX 12"
Box: Subzone Z series port
Wiring: Stinger 4 gauge, Stinger "Bullet" RCA's