I haven't heard the HE-6 from the TOTL headphone amplifiers yet, although have heard them from some really high end speaker amplifiers (McIntosh, Luxman, PrimaLuna, EAR, Marantz reference gear) all over 3k at the least. They are definitely better than HE-500 although I still got a sense of fatigue in long sessions. The thing I hate about the HE-6 is that you get tied down to a single place with a speaker amp. Also, not the best comfort wise in comparison to Audezes, Denon, Fostex and Sennheiser.
They hit hard and in all the right places. Although they lose out on extension to Audeze or Denon. Still very fun though.