Had a K712 for over 2 years going through motherboard audio, an ASUS external soundcard, an E09k + Sanskrit stack and finally an A-GD NFB-11 and each pairing sounded markedly different.

I was (and still largely am) amp and DAC agnostic, but bought the NFB for more headroom to build out my collection at the time since the AKG was my first "proper headphone" and so imagine my surprise when it immediately made a clear difference to the attack and dynamics that I hadn't heard before.

All that is to say, yeah that's a very weird headphone. I've observed the same thing going back to it later, but not as clearly with almost any other headphone. Who knows what they did to that thing in design and production 🫠