I agree, the Koss is softer/sweeter on top. If one wants to argue for Koss then they would say the Stax is a bit bright. IF one wants to argue for Stax then they would say the Koss is too dull.
I find enough high end detail but still listenable.
As far as Koss amp being bad, well, with all Stax amps and Stax headphones I have used I feel I have more distance between me and the performer than with the Koss setup.
In business for 3 decades I have seen the incredible attraction power of "sexy" audio goods for guys and the amazing perception that can result in inferior sounding "sexy goods" being judged superior to cheaper looking little boxes. Appearance and marketing absolutely rule. Price being higher on top of that is better yet for positive impression points. "New and updated" trumps older, even if new and updated is not better or even a bit worse.
Everyone reading this is now shaking their heads saying "no, that does not apply to me, I'm immune from bias because something is more expensive, the new one, known as the best, is machined from billet." We won't get into that debate. But I will say that the status of the Koss electrostatic is hurt by its age, accessibility, low cost, unassuming plastic amp, and lack of marketing and product sexiness.
Better than my Stax? I won't say that. Worse? I don't think I can say that either, except for 007 and 009 (keeping the discussion to current products also). They are worth listening to. I won't present them as the best, but they are real neutral natural nicely detailed natural sounding rigs.
Forgive the rambling, this was written between phone calls so the train of thought is not the most linear.