The HD-600 is excellent for this sort of music, and the HD-650 is also fine for a darker version of the same sound signature (just a little too dark for me, personally). They're fairly relaxing and very comfortable to wear for hours.
Two that I've briefly auditioned were also fantastic for acoustic instrumentation, the DT-880 (semi-open) and AD900.
My favorite sonically for this purpose is the SRH-940. It is closed, but has a reasonably spacious soundstage. However, that and the fact that is is the least comfortable (though not uncomfortable) of these may sway you against it.