Great review bpandbass. These headphones are real gems. I in particular adore the treble presentation which I had never come across with my HD595s which were quite dark sounding. About the midrange you say "This isn't a mid-forward headphone." which I agree with, but on-line there are so many who think these are mid-forward headphones. Presumably this comes from previous ownership of 'V' shaped frequency responses with recessed mids that make them think this. I think the mid-range is as balanced as the bass and treble. Interesting what you say about the sub-bass, I have never found a frequency response curve for the HD580. The HD600/650 have limited bass extension compared with modern high grade headphones. A truly remarkable set of headphones it's a pity Sennheiser no longer make these.
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