Alright so this is what I have right now.. just got them.

Kicker ZX650.4 4CH amp, 650W RMS.

A set of kicker component speakers, only 1 pair.
Its in my RSX... Was told I will only need 1 pair as it should sound really good still.

Anyways I was wondering if I can bridge the remaining two channels on this amp for a sub?

I am only looking at one sub, as I don't want to take up a lot of space. So I'm not worried that I will only get 325W rms or whatever.

But will I be able to?

 

If the amp has a low pass option on it than ya, just put the subwoofer rca's into the rear channel, but make sure the ohm load doesn't go below 4 ohms or else your amp might get a little to hot.

 

What does your owners manual say about bridging two channels on this amp ?!?!?!? That will answer your question.

In the RSX, you are correct. You only need one set of components in the front end. It will sound very good with that setup. Rear fill is a waste.