(snip) But well engineered DACs do sound so similar as to be nearly identical, and any differences that one detects would be splitting hairs. (snip)
The truth of this statement varies greatly with the experience of the listener and the other gear that's present in his or her chain (i.e. from true to not true to everything in between).
There are legions of people who listen to music every day, who most certainly cannot tell the difference between any two "well engineered" DACs, primarily because they've never spent much time listening to "well engineered" DACs with "well engineered" sources, amps and transducers. Depending on the headphone their using, nearly all novices will express some measure of appreciation at first going from their laptop's sound card to something like an Audioquest Dragonfly, but if, on the very next day, they are allowed to hear even a $10,000 DAC with the same headphones, it's highly unlikely they are going to discern any difference. Two things are missing from that scenario - experience and a chain that's worthy of the comparison.
You don't have to possess mythical "golden ears" to hear subtle differences in well engineered DACs. All you need is a few years worth of adaptive hedonism spoiling your appreciation for lesser gear - the process in which you
repeatedly get bored with that which was at first pleasurable and
repeatedly seek a better experience.
-AND- This won't happen unless you've persistently eliminated weak links in the component chain, as you raise the bar with one component, then with another. A single weak link can obscure the differences between well-engineered DACs.
I think we can all agree there is absolutely no point in comparing "well engineered" DACs while using a $100 pair of headphones. Sadly, even a $1000 pair of headphones or a $1000 amp can obscure such a comparison. My LCD-2 Rev. 1 are not known for neutrality or resolution and they have shelved highs. They sound just fine with the most "glaring" of ESS DACs, yet no better with NOS DACs - at least not in my experience. Try that with the HD800. In my opinion and that of many others, it is all but incompatible with ESS DACs, though most appreciative of NOS DACs (and a few OS DACs other than those by ESS). So, in addition to having well-jaded ears, if you're going to evaluate well-engineered DACs, all the downstream components have to be up to the task.
The HD800 is a "microscrope" into everything that's happening upstream, including the quality of the tracks you're choosing to play. It's very sensitive to amps that use a lot of negative feedback. It much prefers zero-feedback or, at least, low-feedback amps. When I want to compare DACs, I plug my HD800 into the zero-feedback Metrum Acoustics Aurix, set to 0 dB gain, to bypass its gain transformers (serving as a passive pre.)