Sounds like you've never heard the HD600 out of some top-tier gear. The HD600 scales incredibly well, along with the best out there (HD800, Hifiman, Audeze, etc.)
Perhaps what you're trying to say is that the HD600 doesn't need expensive upstream to sound great. I agree with that. But better electronics (both amplification and D/A) yield obvious improvements with the HD600.
Oh I've spent extended comparisons on very expensive top flight equipment with my HD600, HE-500 and other upscale cans my friends own. One hears obvious improvements if they are expecting to, that's the way humans work, Auditory memory is very short and we are too easily influenced by suggestion. I have A/B compared equipment using switches and having matched volume levels (very important) and found the most obvious differences due to tubes having more even order harmonics (as expected). Once one has an amp with low distortion, flat FR, proper driving capability for the desired load (cans/impedance) one has reached their listening destination.
Many manufacturers do not properly state their specs or are deceptive. Few if any disclose transient (TIM) specs and many leave of IM specs. IM and clipping are easy to hear. TIM requires more skill, but once you learn to hear it, there is no problem with that. As it stands most quality manufacturers do an excellent job.
A human's ability to hear distortion is not as great as most people think. and most quality amps far exceed this to the point of only benefiting lab equipment and IMO susceptible people.
IMO, unless you need a great deal of wattage for a can like an HE-6, one need not spend a wad of cash unless they are looking for extra features, build quality, styling, bragging rights and so on.
These are my opinions, I am not forcing anyone to think other than they wish. My ears are very well tuned and my imagination under control.