Cadcam,
Really nice mini-review! Congrats on a nice write-up! Like you, I was very, very impressed with my HD600s when I first got them and I enjoying them even more now that I have a Schiit Valhalla 2 OTL headphone amplifier to drive them. Really nice a combination of a neutral, quiet, clean and musical amplifier synergistically interacting with a classic headphone design that still performs at a exceptionally high level today. I've only gotten into headphones at the very end of 2014; I've been a loudspeaker-based 'phile for the last 7 years, and have a very nice, rather high-end 2-channel system driving Dynaudio Contour S3.4 loudspeakers.
The transducer that HD600s reminds me most of are the classic Quad ESL-63 electrostatic loudspeakers, a speaker known for it's transparency, top to bottom coherence, and neutral yet remarkably beautiful and natural tonality. Much like the Quads, you can listen to the HD600s even at low levels and hear deeply into the music. When driven with first-class amplification electronics, ESL-63s, which are probably over 50 years old as a design can still compete with loudspeakers of today extremely favorably. The HD600s are very much like this in that regard.
You've probably read that HD600s "scale" beautifully with better and better source and amplification electronics, and they do. They also scale with better power cords for the head amp and DAC and higher quality interconnects. They also scale, however, with better and better source material. They not only sound good with Redbook level 16/44 content, but drive them with exceptionally well-recorded and mastered higher res content, and they can be quite astonishingly good.
One can always have more bass, more resolution, more sound staging, more this, more that, but at the end of the day, HD600s do more things better than almost any headphone I can think of, and most importantly, they create an an extremely engaging and beguiling listening experience. The fact that you can get this typically for around $250-$300 is a remarkable value proposition.
And like you, I haven't been listening too much to my main loudspeaker-based system very much of late.