98 V70 XC audio system and aux/3.5

[CIK7]

Ive heard that the sc816 hu is pretty much the vanilla flavor for the 70 series p80s. i know they come in the 98 xc models (1.) because i have one and 2.) because ive seen em. I've had the entire interior apart and I'm assuming there is no external amplifier. I read somewhere that the factory built in amp is 50Wx4. This true? When I took out the tweeters, they said 8ohm. I found a great deal on some top end speakers and a sub. Would like a 5 channel system w/o the cost of a 5 channel amplifier. Not sure if I'm gonna follow through. There are always more important things to be paying for.
The speakers I found are 4x75W RMS and sub is single 250W RMS. I would like to know:
Can I setup a single sub on all 4 factory channels (for 200W of power)?
Is this an 8ohm hu?
Can it handle 2ohm or 4ohm setup?
How should I go about setting up a 5 channel?
Hypothetically, can a home theater speaker set sound better?
Lastly, I love using my device and picking the next track. Anyone have a walkthrough of CD to aux conversion?