[rant]
As much as I like the SR-404, I actually prefer the HD600 over it for listening to piano recordings, and that probably should tell you something.
Piano's not a texture-dependent instrument but it is very tone and tembre dependent, and a headphone must get tone and tembre right in order to make piano sound believable. Also, a headphone must needs have sufficient impact to communicate the piano's percussive nature. Lastly, it cannot have a recessed midrange or any coloration in the upper midrange, since piano will make any such colorations and tonal imbalances painfully obvious, whereas other instruments will not.
The HD600 doesn't do textures very well, and it doesn't resolve the lower bass registers quite as well either. It's upper treble is a bit recessed, and it's lower treble is a bit steely. However, as far as piano goes, none of these flaws really become evident. The headphone has a very good tonal balance overall with nice rich mids, which give piano a lot of body, and sufficient detail that the reverberations of the strings are evident, yet not overbearing. Also, it has very good impact, which translates into a forceful, realistic rendition of the sound.
The only problem here is amplification. You need something that's beefy enough to open it up; without sufficient juice it will sound murky and closed.
The SR-404, by contrast, is vastly superior in terms of textural portrayal and resolving power, but it has somewhat recessed mids, a slightly colored and sibilant upper midrange, and an overall lack of impact. That's not a good combination for piano at all! Just goes to show you how a lower priced headphone can substantially better a higher priced headphone if the sound signature is exactly what you need. With strings or woodwinds or anything where texture is more important than tembre, the SR-404 wins hands down, and I prefer it in general to any dynamic except the R10/010, though this is of course subjective. I think the HE60 will get both the resolution and the tonal balance right, as will the SR-404 with an all-tube amp amp. I bet the Omega 2 would be the ultimate headphone for piano, since it's quite impactful and very well balanced as far as electrostats go. The HE90 that I've heard did piano better than anything else that I've experienced to date but that is, well, an HE90. So...
I haven't heard the DT880. I prefer the HD600 to the 650; the 650 has more resolution but the 600 somehow has more air and space, and a tad more impact.
[/rant]