Well for a planar magnetic, a great treble is somewhat of an anomaly,while bass and midrange usually comes with an expected excellence, imo of course. Planars seem to struggle with resonance issues that can plague the treble, and that has been my experience as well.
Headphones like HE-400 have far too much treble, Mad Dogs have too little of it and it's too unrefined, LCD2 has both too little of it and a slight hardness around 9khz. PM-1 has a rough transition from upper midrange to treble. LCD-X has too much of a spike around 9khz-- an accentuated LCD2 problem imo. I've not heard the HE-4,HE-500 or HE-6 but I've read from trusted ears that they all have issues with treble as well.
HE-560 (with focus a pads) is the first planar I've personally heard that does treble right, the balance at around 10khz is just right, and the transition from upper midrange to lower treble and then upper treble is near flawless, it remains relatively smooth while having enough presence in that region to keep a linear sound. Other planars have dips or peaks in that critical transition from 4-10khz.
How 'bout the Alpha Dogs?