Although the DT 880 doesn't change its character as much as the HD 650 with break-in, «over a dozen hours» is by far not enough to hear its true potential. Give it 150 hours before making a judgement. That's not to say the sonic balance will alter noticeably*, but it sure will do better what it does now.
That said, the HD 650 was the reason to sell my DT 880, which I really liked during the time I had it. As others have stated, both headphones share some similarities, and to my ears the HD 650 is better in most criteria and as a whole the clearly better, more accurate and musical headphone. The only thing the DT 880 does better (IMO) is bass extension. It has the flatter bass response, thus less drop-off with ultra-low frequencies. At the same time it also has a leaner, less «breathing» and less impactful bass. The DT 880 doesn't have a completely open back, and I'm quite sure the relatively small vent in the center of its rear to let the soundwaves escape doesn't allow a similar effortlessness and airiness in the lows as the fully open HD 650 shows. Nevertheless, the DT 880's bass has great definition and accuracy, it just lacks the impact, cleanness and naturalness of the HD 650's. Talking of the latter's «mid-bass hump»: This phenomenon which is quite prominent with a new unit disappears more or less with driver break-in and headband loosening. Although I agree that the DT 880 still shows the flatter bass response, but at the price of less atmosphere and naturalness.
The midrange is excellent with both phones, smooth and liquid -- I don't understand the comments about the Beyer's «dryness» --, just that the DT 880's appeared as a bit recessed because of the dominating treble. The DT 880 is the first headphone with the latter feature that I liked and could listen to without fatiguing. It's smooth and resolving and extremely extended. After owning the HD 650 for a while I must concede: a bit too extended -- too sharp to be absolutely natural -- for my taste. The HD 650 in comparison has a silky-smooth, sweet and at the same time very focussed and accurate treble with even better resolution. With the right setup, it's hard to beat when it comes to reproduce the natural sonic colors of a classical orchestra. I never got the same authenticity with the DT 880, although it was really enjoyable and musically convincing with classical music. It may show greater detail than the HD 650 because of its outstanding treble, which may (or may not) be favorable for other genres, but the HD 650 has the clearly better resolution. IMO it's a class above the DT 880.
But keep in mind that the HD 650 is «harder to drive» (to use a common wording) than the DT 880, despite its higher efficiency. The reason is that it's sonically demanding. Because of its flat frequency response, it may sound dull or uninvolving in some setups. And a good aftermarket cable is mandatory to get the best out of it (as much as some may hate this statement), which raises the costs considerably.
* [edit:] Actually the sonic balance will change quite a bit to less brightness and fuller bass with worn-in earpads which allow the drivers to get closer to the ears (= exactly the reversed effect of the HD 650's headband loosening). But it takes at least 500 hours for this.
It's just that when I find something this good, I just want to spread the word and hope others can experience the same joy.
