The headphone outputs are often limited to a certain level to prevent the user from hurting himself by putting the volume up... As was said, look for something with a legitimate line-out port.
Make sure you're looking at the RECORDING volume controls and not the PLAYBACK ones. In Windows O/S's you need to go to the Options > Properties menu in the volume control applet.
I have this problem, but only when I run the scanner from batteries. It seems to halve the output level when there's no wallwart power source. Just adding this in case your scanner behaves the same way. I'm about to buy an isolation transformer so I can use the wallwart again, because I'm getting volume complaints without and a nasty buzz with!
Ok, hooked up from scanner headphone jack to line-in...playback and recording levels were/are up all the way, on wall power. Tried moving all left or all right, didn't work, so adjusted still in middle