Because of ASR and Solderdude's DIY Audio Heaven website, I've got a new headphone system. I noticed that the DAC and headphone amp have distortion levels that, in theory, would have no audible impact on the sound. That would be the Topping E/L 30 pair. The source is a mini flash drive full of Apple lossless files plugged into a cheap laptop. This is all complicated by the use of the APO EQ, enabling me to jack up the bottom octaves beyond taste or reason. Good for the Material track I'm listening to now. The biggest variable is the headphones. I'm using Drop 6XX 'phones now, will be getting Philips X2 HR headphones later in the week.
About 30 years ago, I was using Stax earspeakers, and in between I've had lots of different headsets, all sorts. The Stax were the best for the subjective sense of "resolution", but there is a quandary in that the sources available at the time were not high resolution. Back then, my LP playback gear was pretty good but far from the best, and [as we all should know by now] the LP format is all over the place as regards sound quality and, in the case of used LPs, wear. At the same time, digital gear was, on average, pretty crappy at the time. So I don't really know how good the Stax headphone/amplifier combo is compared to what I'm listening to now. The upper registers via the 6XXs seem a touch muted, but I have to wonder how much of my expectation of more 'grip' in the treble is based on listening to distorted sources. I am hearing the sorts of things subjectively described as more voices, more musical lines, more 'inner detail'. I am not hearing a lot of the congestion I recall from playing tracks previously. A lot of those memories are from LP sources, so a lot of what I recall is obvious distortion. "The Chain" and "Go Your Own Way" from Fleetwood Mac's "Rumours" are good examples, both wandering into clipping on peaks in the LP playback, sounding congested on top during the vocal peaks on CD playback from years ago, open, clear and harmonious playing from my rip or from Amazon music.
"Resolution" in audio is not as definite a term as it is in video. Get close enough to the screen and it's easy to see how the difference in measurable levels of resolution affect the image quality. But do the same with audio and a 'lower resolution' recording, and the real difference usually turns out to be a difference in mixing or mastering, not a jump in the amount of data per second.
Maybe "resolution" is a red herring.