That's a fair point: no, I didn't hook up headphones while recording, for two reasons:
1) I wanted to test the DACs more than the attached headphone amps
2) Headphones would have altered the recordings to a certain extent (frequency response due to high impedance, stereo crosstalk, and possibly some level of distortion), but different headphones would have had different effects, so nothing close to a common ground could have been met.
Ha! That didn't take long. Are we claiming that all DACs sound the same now, and that it's the headphone amp that makes all the difference? That is not what I've been told again and again.
Well, people listen to DAPs with headphones. Hard to say how they are going to sound as sources unless they are using radically different digital filters though depending on how they output a signal there might be subtle differences. No two pieces of electronics that are different will perform exactly the same, but it's ultimately going to be up to people to decide individually if any difference is worth caring about, either empirically or subjectively.
There's a lot of technical discussion that can be had about it. For example, a slow-roll-off filter versus a brick wall filter might be audible to people who can hear high frequencies with certain music, but in a DAP where most people are likely to listen to MP3s where everything about 16kHz has been cut off already the filter likely wouldn't make any difference. Even then, we'd have to consider what music we're listening to, especially on Head-Fi where there are plenty of people who fill their DAPs with lossless files and use better than average IEMs or headphones and value hearing exactly how many millimetres the drummer hit the cymbal from the edge whether or not they can hear the nostril hairs of the artist swaying when they breath and so-on and so-forth so they will care about the tiny differences to the point of overkill sometimes.
I think it is interesting and valuable to do listening experiments, but for the sake of fun and learning rather than trying to prove a point. It's supposed to be a fun hobby after all.