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

Ideally you want the tape output. 

Best,

Erik 

I think you're right. Have to man up and get that preamp out!

Make a digital copy from each output.

As in: preamp output, tape or line output, and headphone output.

In an ideal home stereo listening post (in chair listening to stereo), each will sound different.

In each piece of gear, the results, or sound qualities... of each of the three -will be different.

Meaning, the lessons learned from finding the best ’tap’ point for a signal on one piece of gear..... is not necessarily translatable to the the next piece of gear.

In audio design, there is no specific or normalized way of implementing any of the three, but merely guidelines. Each designer may do it similarly but each will not do it exactly the same way. Some may execute those functions completely differently than the next designer. Which will beget a varied set of results.

The tape output or line output (the same, generally) of a preamp....is generally a straight unamplified, unmolested ’take’ from the phono preamp circuit itself, but there is no guarantee in this... but the odds are high. Usually one buffer resistor is in play on that output signal, internally (inside the preamp). Something like 220ohms to 1k ohms.

How one hooks up the tape outputs for using the phono circuit on said tape outputs can slightly change the sound qualities of the phono RIAA preamp circuit, which is part of the reasoning behind why most preamps of higher quality have a ’tape off’ setting, or ’rec out’ off position. (Example: Yamaha C-4 preamp)
 
The Adcom in question has a opamp on the headphone output, IIRC, and is thus a quality step down from what can be achieved via the tape output, or even the preamp output - possibly. It is minimally built and built out of opamps, so it has it's limitations.