Or maybe it's just the way they are made?
http://gearburn.com/2014/04/logitech-z200-review-stylish-speakers-stifled-sound/
".. but dial it too far at high volume and massive distortion prevails"
You do need the computer OUTPUT VOLUME to be high enough as well or trying to amplify the signal is too hard. I would set it to about 50% by default at the PC with the knob near the LOWEST then raise it up until it's proper volume (or distorted).