Speaker rating question

Cut off the internal amp in the headunit on the Alpine if you have that feature. That will clean up the sound and then add an external amp and leave the speakers you already have.

Since your speaker are already overkill for use without an amp, you need to get a 4 channel.. In general, if you wanted to be cheaper and run off deck power, get a speaker which is more efficient meaning lower power numbers... This is a ridiculously common mistake... People figure the higher the power rating on a speaker, the better it is.. FALSE!!!!! I run memphis m class coaxials in my Explorer and they outperform many amped systems because they are so efficient.. Carbon fiber cones, rated at 50 rms, 100 peak.. They even have very significant midbass..

You have to remember a few things:
1. Even though the headunits amp gives a rating, that rating usually has a high level of distortion.
2. I'm sure you've seen a "true" 50Wx4 amplifier. Remember how large of a chassis it took to fit the components into it? Now imagine the size of the amp in the Alpine deck and you can understand why they aren't the same. There's now way they can cram quality components into that small a space.

IMO, your decision to have "better" sound is vague and you need to ask yourself a few questions...

1. Do you want to be able to play it louder and still have it sound clean?
2. Do you mean better in terms of frequency response of your system?

The external amp will give you the ability to play the system louder without distortion, meaning "less" chance of damage to your speakers. However, if you want better frequency response, i.e. bass, your only option is to add some type of subwoofer(s) AND an amp for your 4 other speakers. No matter what you do, your 6x8's won't produce any low frequency notes. Turning up the bass on the deck will only add distortion, meaning the speakers cones are moving more than they're intended to do, and eventually you'll throw a voicecoil and have a blown speaker.

I can go on and on, but hopefully you get the idea. Hope that helped.