I got my pair of HE-6s late in the game, when I already had the Theta V A which is an R2R DAC thing. You can already see where I am going with this. The treble never presented any problems. I anticipate as the DAC game changes at accessible budgets with the Yggdrasil/Multibit Gungnir we will see much less of this treble problem. Just avoid horrible etched amps and I anticipate we won't be talking for much longer about this.
The HE-6s are very resolving headphones. On this note the HE-6s (and HD800s) just keep scaling and scaling, and that includes resolving power.
With this DAC animal I am auditioning, the HE-6s are reaching previously unknown heights in how well they can perform. The treble is awesome too.