Listen at different volume levels. I find the HD650s start to sound more precise if you set the right volume for the specific recording. They don't suffer from a lot of ear-generated distortion at higher volumes, but they also, I find at least, are crisper and more detailed at lower volume levels. Sometimes if you have a very warm recording, as Waits tends to have, it actually sounds better a little louder. More midrange presence. Hard to describe, just encouraging you to experiment.
I'm sorry, Copperears, but I don't think that I have acknowledged this. How very right you are! The Senns are very volume sensitive. If find increasing the volume useful more often than reducing it, but what do I know? I have only burned these cans in for 15-20 hours or so and listened for 4-5, maybe. Anyway, your insight is very valuable, thank you so much.