Let me see if I can help out a little...
The specs on the amp don't look right from the link you posted. One line says it's a two channel, another line says it's a 4 channel. The rms power ratings don't look quite right either. So here goes.
The amp you picked out is a 2 channel amplifier.
Ideally, each channel would drive one speaker, giving you "stereo" mode.
Example: Channel 1 goes to left front speaker, channel 2 goes to right front speaker.
BUT, you want to power a subwoofer with that amp.
This is possible, BUT you would need to "BRIDGE" the 2 channels into 1 "Mono" channel.
This is usually done by wiring the (+) lead of the sub to channel 1 (+) ,and then wiring the (-) lead of the sub to channel 2 (-). Doing this almost doubles the power output of a single channel. For example, that amp says 125w per channel, so bridging would give you ~200w to your sub. Does that make sense?
You also have to take into consideration impedance loads (ohms).
Look again at the specs on that amp. It will do 125w per channel at 4ohms.
Usually, the lower the ohm load, the more power an amplifier will produce. Notice how the power output goes up if you drop to a 2ohm load.
BUT, remember you would have to bridge that amp to power the sub you selected.
Here's a good rule of thumb- amps that are cheap will usually only be 4ohm stable when bridged. Sorry, but that amp is kinda cheap. No offense. You only have one wiring choice.
SO, you will have to wire that sub to a 4ohm load.
How did I just do that?
Well, the sub you selected has two voice coils. (It's kind of like 2 speakers in one... kinda) Each voice coil has an impedance of 2ohms. If you wire the voice coils in parallel, you present a final load to the amplifier of 1ohm. (Parallel means (+) to (+), (-) to (-). )That amp would instantly go into protect mode. It might fry if you tried it.
The way you would have to wire the voice coils would have to be in series. (Series means (+) to (-).) By wiring in this manner, you present a final load to the amplifier of 4ohms. This is acceptable for that amp.
Still with me?
Check out the Woofer Wiring Wizard at the Rockford Fosgate site for help wiring. CLICKY!
I think you're off to a pretty good start here. Personally, I'd look for a mono subwoofer amplifier so you could have more options wiring subs up. I also believe that while that amp could push that sub, id be nowhere near strong enough to drive it to its full potential. That sub has an RMS rating of 400w. You'd only be giving it ~50% of what it could do. I think you'd be disappointed in about 2 weeks.
Check very carefully about that amp. I'm pretty sure it's bridgeable, but make sure before you drop any coin on it.
I hope I was able to help you out. Maybe some other member will chime in.