I'm currently making a custom sub enclosure for the back of the '05 X and want to run the amp/sub off of the OEM Headunit until I find the HU that we want... Is this possible to do? If so, which wires need to be spliced into? I'm assuming i need a switched power lead and and lead from R/L speakers... Correct?

there's a unit you can buy from a place like circuit city, best buy, or crutchfield that you wire into the speaker wires with gives you a set of RCA connections to plug an amp into. there are also some amps that have a speaker input that just need speaker wires for a signal. either would work.

Is it clean sound? I assume it would be much cleaner with an aftermarket HU?

it's an alright sound, no where near an aftermarket HU, b/c you gain a lot of level controls, bass controls, and sub controls with an aftermarket HU where as a stock only usually has treble, mids (maybe), and bass.

the device discussed is called a hi-low converter

i have this Hi-Low converter on my alpine unit and works fine
its made by
PAC

edit:.. PAC SNI-35 ADJUSTABLE 2-CHANNEL LINE OUT CONVERTER