I don't have your gear, but I'm pretty sure you should put the volume on you computer up to full, and on your DAP I'd try 2/3 to start and experiment with going up/down from there.
There are variables that could change this, e.g., which player software you are using on your computer, soundcard firmware configuration, etc. For the portable, I don't know if it has a true line-out option or whether you have to use the headphone-out. If there is a way to get a true lineout rather than using the headphone out (whether via firmware configuration or through a compatible adapter), that's the way to go.
Hopefully, someone familiar with your particular equipment will step in. If not, start with the suggestions above and use your ears to make adjustments. Hope this helps a little.