Well, this ends day five with my DT880s.
I've been taking a look at the HD595s, which seem to be the only other headphone approaching this degree of technical performance from the bass through the mids, with the Beyers seeming (by the headroom graphs) to hold a slight edge in bass extension and midgrange smoothness. What seems to distinguish the HD595s from the Beyers is that the Senns run about 10 db down compared to the Beyers in the low-to-mid treble. Even though the Beyers are a little in excess of neutral in the treble, I don't think I'd want to go 10 db down with the HD595s. They both seem to have a spike centered at about 9 khz... the Senns just shelve down very quickly after 4 khz to accomodate the spike. In short, both of these superb headphones are a little rough in the treble. The Senns just do it in a subdued way, while the Beyers keep everything else quite neutral but just let the spike at 9 khz stick out there. I think uneven treble is just part and parcel of over-the-ear headphones and how they interact with the surface of the ear. I don't think the state of the art has gotten past that.
Plus having tried the HD595 at meets I think the Beyer build quality is a little better than that of the Senns. The Beyer headband is metal and there are metal screws helping to keep things together. It's hard to imagine them falling apart. I have the Senn HD580s and HD280s and the build quality does not seem as good.
With my digital EQ I think I am getting very close to a neutral transducer with the DT880s, so I get to hear something pretty close to exactly what's on the recording. That's pretty exciting.
I wish there were a headphone with the DT880s level of performance from the bass through the mids but also with a treble performance to match it. I don't think there is.
I should emphasize that the Beyer DT880 "brightness" is not a midrange brightness but a treble brightness. The two are very different. Grado SR60s and Sony V6s by contrast have a midrange brightness, which can sound brash. I compared them today and the midrange on the V6s and the Grados is definitely brighter. Treble brightness (which the DT880s have) results in a brilliance or sheen and emphasis on instruments like triangles, drum cybals, etc. Listening to jazz, the DT880s remind me of being in a very small club (a situation I have been in many times). They are definitely going to reveal ambiance and detail in recordings in a way that is very exciting and engaging, as well as emhasize percussion instruments that involve the upper end of the audible spectrum. Also, the elevation in treble lessens the apparent level of the bass by comparison. If you wanted to go down to neutral without smoothing things out with more precise EQ, you could more or less get there by turning a conventional treble knob down about 4 to 6 db, I think.
So, in truth, I am a little disappointed by the treble emphasis in light of the absolutely brilliant bass and midrange performance of the DT880s. I think I have entirely compensated for it with digital EQ. The DT880s are basically linear transducers except for the audible spike in the treble. I just can't help but wish that there was some headphone that took things to the next level and got the treble exactly right, but I just don't know of any such headphone that exists right now. I thought closely about the HD595s but they seem to have a very similar problem in the treble but 10 db down... sort of shoving the problem under the carpet, if you will. Mind you, these are the two best technically performing headphones that I know of, in terms of what I am looking for in a headphone, bar none. You definitely get your 200+ dollars' worth with either one.
Thus ends my thoughts at the end of day 5....