For what it's worth, when I first heard the DT 860 I thought it was
unbearably bright with a strong lower highs emphasis (maybe around 2-3 Khz) that made everything sound cold and overly shiny. Even driven by the Corda HA-2 MKII's 120 Ohm output, the brightness was still overwhelming and at that time there was no way for me listen to that headphone any longer. I thought the DT 860 was one of the worst audiophile headphones I have ever listened to. It should be noted that a couple of other headphones were present for a comparison, namely a Hd 650 with various replacement cables and two Grados (80 and 225).
In the meantime, I have bought a Beyerdynamic DT 660 which inherits the same brightness issue, which I believe to have encountered with the DT 860 before. In addition to that my Porta Corda MKI broke down so all I had was my portable player, a couple of closed headphones and the DT 660, which made me shift my home listening to these closed headphones. I have to admit that I've grown quite found of the DT 660's bright presentation despite or maybe because of the lower treble emphasis. On some tracks, the DT 660 is still unlistenable but most of the time I find the Beyer to be extremely fun to listen to. After some time I also bought a 120 Ohm adapter from Dr. Meier, which made the DT 660 even more enjoyable. Although the DT 660 is not the DT 860, there are certain parallels. ssportclay mentions a groovy sound and I can concur although it is not the typical warm, upper-bass humped, midrange-emphasised stereotypical groovy sound but one that could be described by the term uplifting. It's the sort of sound that grabs you at your neck and literally screams for your attention with a rather bright voice. Please also note that the lower treble emphasis does not necessarily lead to listening fatigue (at least for me) as is the case with the Ultrasone HFI 650, which exhibits a mid treble emphasis at around 5-6 Khz.
I'd like to give the DT 860 another listen but I guess the bottom line is that - assuming I can make assumptions on the DT 860 based on prior experience with it and my personal observations with the DT 660 - the Beyer 860 definately requires quite some mental burn-in and a preference for brighter sound signatures. And as has been mentioned already, a 120 Ohm doesn't hurt with these headphones. Nevertheless, if you're looking for an even-handed frequency response, look elsewhere. Certainly, there's no "perfectly neutral" but I consider most audiophile headphones to be somewhat neutral or rather balanced across their respective frequency responses. Both the DT660 and DT860, however, have too obvious frequency bumps in the lower treble to be called anywhere near neutral. Take either headphone, listen to some white noise and you know what I mean. (As a sidenote, comparing headphones using white noise or pink noise can tell quite a lot about a headphone.)
Sound quality aside, the DT 880 is much more comfortable