Is preamp's headphone output as good as "true" output?

I admit I'm being lazy here...   My setup wasn't designed well for accessing the rear panels of components. I want to make a WAV file from an LP, at good quality. The front panel of my adcom GTP-500 II preamp has a headphone output that has no volume control, so I'm wondering if I could send that signal to my computer for the conversion to digital. On the rear panel (if I go to the trouble to pull the component out), there's probably a pure, absolute output for such purposes, but is it different from the headphone output?
While I'm on the subject, any suggestions for a simple-enough and accurate conversion of the analog signal? I assume what I need is a PC app.

Thanks!