Thats a nice little amp. Let me give you a brief explanation about impedance.
Lets say we have two 4-ohm Single Voice Coil (SVC) sub-woofers, you can wire them in series like so: http://www.rockfordfosgate.com/rftech/woofer_wizard/2_4ohmSVC_8ohm.gif
to get a final load of 8-ohms. Your other option is to wire them in parallel to get a final load of 2-ohms like so: http://www.rockfordfosgate.com/rftech/woofer_wizard/2_4ohmSVC_2ohm.gif
Wiring sub-woofers in series just adds the impedance, so sub1 = 4-ohm and sub2 = 4-ohm so series wiring is 4+4=8.
Wiring sub-woofers in parallel will always make the impedance smaller. The formula is
1 / [(1/sub1) + (1/sub2) + (1/sub3)...etc]
So in this case it is
1 / [1/4 + 1/4] = 2-ohms.
Now we have sub-woofers that come with 2 voice coils, properly named... Dual Voice Coil (DVC). This is basically for more wiring options and greater power handling. A 4-ohm DVC sub-woofer can be wired to have a final impedance of either 8-ohms, or 2-ohms. A 2-ohm DVC driver can be wired to have a final impedance of wither 4-ohms, or 1-ohm.
Now that I hopefully did a decent job at explaining wiring options. I want to tell you about Image Dynamics IDQ and IDMAX series sub-woofers. I run a single IDQ10D2v3 (10", IDQ, 2-ohm DVC, version 3) in my car on 500 watts RMS @ 4-ohms. This is the cleanest sounding sub-woofer I have EVER heard in my life, very smooth, clear bass. But, they are slightly more expensive than your average sub.
I would suggest to you, get an IDQ 12 D2 v3... wire it like so: http://www.rockfordfosgate.com/rftech/woofer_wizard/1_2ohmDVC_1ohm.gif
and set your gains so that you are only feeding it 500 watts @ you max listening volume. You will love the clear bass.
Or if you have the money, I would get two IDQ 12 D4 v3's... wired like this: http://www.rockfordfosgate.com/rftech/woofer_wizard/2_4ohmDVC_1ohm.gif
and set your gains so that you are only feeding it 600 watts @ you max listening volume. 10's would work too if you want to save a little bit of space.
If you want to play around with wiring configurations heres the site I was using to get those diagrams: http://www.rockfordfosgate.com/rftech/woofer_wizard.asp
Heres more info on the IDQ subs:
12 D2 V.3.pdf
12 D2 V.3.pdf
Heres an internet authorized Image Dynamics Dealer:
Heres info on setting gains properly: http://www.thesuicidaleggroll.com/gain.htm
(use method 3)