I've been listening to the HD800 for the last 3 hours or so, and out of the box, I'm impressed. The lineup I'm using is computer as source, coaxial to the HRD-Ultra (with DPS). I have the gain at medium, and the volume at 4 o'clock. With my other cans, it's usually at 1:00 or 2:00.
(Correction 6.13.09: The volume setting on the HRD-Ultra is at 12:30 instead of 4 o'clock as stated earlier. When I originally posted, I had been listening with the source volume far short of max and therefore had to crank up the volume on the amp.)
In the SQ, nothing sticks out as irritating or somehow wrong so there's absolutely no fatigue after 3 hours. The fit and comfort is perfect from the get-go. No need to constantly adjust. I've been listening to my favorite music and haven't had a moment when I felt the need to turn to another lineup for comparison. This tells me that the sound that's reaching me isn't wanting.
Trying to put my finger on what it is about the HD800, two thoughts come to me. One is accuracy. Like no other cans I've heard, it allows a clear "view" into all the elements that make up the sound. And this clarity extends into the bass, allowing me to "peer" into it to hear it with absolutely no coloring. Thus, the bass can be soft yet clear, well-defined, and powerful. In other words, the HD800 makes me realize that the bass doesn't have to dominate with volume to be there. It can be just as dynamic when it's relatively quiet. For the first time, I could actually feel my wooden desktop vibrate from the bass as it passed through my arm to the desk. Yet the bass was relatively quiet in terms of volume. The HD800 seems able to tap into the multiple dimensions of bass, and it delivers energy through more than just volume. I think I'm seeing the complexity of bass for the first time.
The second thought is dimensionality. The HD800 is truly 3-dimensional in representing sound. The different elements are separated and defined in all 3 dimensions so each is uniquely positioned and pinpointed by the ear. But what's crazy is that the dimensions shift and the elements move according to the demands of each passage, creating a very real sense of a dynamic stage.
With the HD800, I don't find myself wondering if my other cans or lineups are better. It's like being prepared to experiment with different arrangements, but once you sit down to listen, you don't change anything and just end up enjoying the music.
So how good is the HD800? It's not so much what's good as what's not bad. I think head-fiers are fine-tuned to hear problems in SQ. With the HD800, I simply don't hear any problems. How good is that? In my book, nothing has come this close to being problem-free.
Re the HR Desktop Ultra -- I think the HD800 brings out the strengths of this amp that other cans just haven't been able to.
Re the brightness that some have mentioned -- I think it's more a function of accuracy than coloration. We're just not used to having the mids detailed so accurately. The result is that the HD800 makes your music come alive in ways that are unexpected. It's as though you hearing all the nuances for the first time.