I think this discussion has gotten bogged down in details like extension and midrange colorations - all worthwhile topics, but not addressing the whole musical picture so to speak.
I listen primarily to classical music at home, and I've been through a number of different dynamic headphones and headphone amps, with top-quality sources and source material. I got my Stax SR-404 in late December and they were a *revelation* with classical music, especially orchestral music.
Stax headphones immerse you in music. You are completely surrounded by it. Moreover, it's transparent - you hear layers upon layers of orchestra textures and colors. At the same time, none of these layers and none of this detail is etched or fatiguing. It's sheer music.
The extension issues and colorations discussed above may actually exist, but to me, they're irrelevant. To even bring the K-501 or the HD-600 into the discussion boggles my mind. I enjoy those headphones perfectly well, but they do not put your head in a musical space the way my 404s do. (I haven't heard the Senn electrostats to compare, and I haven't heard the O2. I do have a pair of the SR-X Mk III, just arrived, but haven't had a chance to audition.)
I put my headphones up against a pretty punishing non-headphone rig. Until I got the Stax, there was never any contest - even with a fully tricked out Singlepower amp with every kind of NOS tube known to man.
If you're a classical fan, you owe it to yourself to try Stax as soon as you possibly can. I'm rediscovering my classical CD collection - thousands of them - pretty much unplayed for the past decade while I focused on vinyl. This has been staggering, and staggeringly fun, too.
Patrick