Honestly after owning the HD 650 for many months now and also owning the HD 660S2, HD 660S, and HD 600; I realized this is actually my favorite headphone of the lineup as a whole. There are things I like more about the other 3, but the HD 650 won out in the long haul for me and of the lineup does resolve the most information in the midrange. The HD 600 is great to me to listen to but a bit rough in the upper mids, if it wasn't for the rough upper mids I would actually like the HD 600 more than the HD 650. The HD 660S (late production, early productions sound quite different), is slightly muffled compared to the other 3, and is boring to the point of being depressing to listen to. The HD 660S2 while technically quite impressive is too relaxed in the upper mids and something is slightly off with the treble and something about the bass is a bit overwhelming to me (and I have some bass-head tendencies).
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