This should be a simple question

I am on my computer listening to music through windows media player 11 and it has its own volume control, then xp it self has its own volume settings, then the audio goes out and into my (home stereo, not car) and that has its own volume.. lol

My diagram..
wmp=volume -> Xp=volume -> Stereo=volume Speakers..

To relate this to a car sayy..

wmp=volume -> Xp=volume -> Amp=amplifies Speakers..

Should i keep xp at max volume/wave then adjust in windows media player.
I have also noticed that the volume goes from 0-100 but it is a pretty small volume control, Is it possible to code a head unit that goes by half increments or even quarters or 1 volume .(for Fine Adjustment in Volume Control) ,,,In a head unit app it will run the full length of the bottom of the screen, small in height,,,
Another diagram,
wmp=volume -> EQ=settings/volume -> Xp=volume -> Amp=amplifies Speakers..


If any one understands this please reply.