I'm liking the EF-6 quite fine, thanks. A more powerful and controlled lower end, a more effortless (almost relaxed) upper end. Midrange bloom is cut down a notch, which is welcoming because piano is actually listenable now. The wow-factor is the degree of articulation that EF-6 is able to whip HE-500s into performing.
Great description jerg. That's pretty much how I feel when giving the HE-500 lots and lots of headroom. With lesser amps the midrange is too prominent.