You have a very very good question.
All amplifiers have an optimum volume they sound best at. Typically it's around 50-70% volume control.
So you have to experiment. Find your normal volume level (that your can't at the moment as your running in speakers!).
Then take the windows volume down, and slowly increase the volume on the amp. Then try the opposite taking the amp volume down, and windows volume up.
Eventually you will find the optimum level the amp volume should be set (for best quality), then just leave the amp's volume fixed and control your listening volume with windows only.
But as mentioned you should not be doing this until your speakers are run in.
