Update: gave the headphones a good critical listening session this time. Since I got in the HD600 on Friday, I threw it in the mix as well, for a comparison, since it's the best-known can of the three.
Equipment chain has changed again!
NAD T533 > Signal Cable Analog Mini > Little Dot Micro+ > headphones
K701 is nearing 500 hours.
DT880 is nearing 400 hours.
HD600 is unknown, bought it used. Apparently ~100 hours.
Little Dot Micro+ is somewhere around 400-500 hours.
Note: HD600 was tested with the Blue Dragon re-cable.
Instead of doing CD comparisons this time, I'm summing up the overall signature of each.
The HD600 with Blue Dragon re-cable proved to be the most stubborn headphone in my testing. Not that it was bad, because it was awesome...on any music remotely resembling acoustic. It fared much less well with electronica and some of my trip-hop. Bass extension and texture wasn't even close to the DT880's, leading to a lack of satisfaction with primarily The Crystal Method and Massive Attack. On the music it did do well on, there were excellent dynamics, relatively neutral overall tone, with sharp definition. Very swift attack, with a good decay, although the attack doesn't go over too well with higher-frequency notes, leading to a literal sonic mess. Very noticeable on certain Orbital tracks. On cymbals, it didn't give razor clarity to multi-cymbal swishes. Probably the "flattest" sound in the round-up, at least up to the highs. There seemed to be some kind of spike up there, as attacks in the highs (notably cymbal impacts) were a bit excessive (though nowhere near as intolerable as in the Sony sound signature). Overall, it gave the most realistic sound.
The DT880 gave a GREAT middle-ground between the HD600 and K701. First of all, the bass. It was nearly non-existent out of the box and leaner than the K701's for a LONG time. Only recently did it bloom into a full-bodied, partially visceral slam. On tracks that feature textured bass notes, the DT880 clearly reproduces them. Extension and impact were significantly more than on the HD600 and K701, making for an enjoyable listen with electronica - plenty of boom, and enough extension to get the upper part of bass phases and rolls designed for subwoofers. (However obviously not as much extension/impact as the DT770.) Overall, the lower-frequency register carries a TAD bit more power than the mids and highs, but somehow the mids and highs still manage a decent balance against it. As for those highs, easily the best in the round-up. Captured the most raw detail, like the initial "sizzle" impact on cymbals (which even the HD600 routinely failed to get) and other similarly super-fast sounds. However, there was one big fatal flaw in its presentation - its attack was noticeably slower than the HD600's and K701's. Not so slow that it could be called sluggish, just slowER in comparison. On the bright side, it had the best decay - nice and full, which allows it to get the latter part of a cymbal crash better, and effects with echo and reverb, and allows percussion instruments to sound real. In fact, it gave the fullest sound on the belly drum in Massive Attack's "Inertia Creeps." Easily the most "detailed" presentation of the three, but at the expense of attack, and power in the highs, as the highs overall sounded a bit weak relative to the lower frequencies. Of the three, it gave the best performance on Massive Attack's "Mezzanine" CD. Other than the attack issue, and a slight grainy edge to lower-pitched female voices, it was hard to find faults with the DT880, even though it didn't have the realistic edge of the HD600.
Unfortunately, the K701 was the most disappointing. To its credit, it does do TWO things extremely well. The first is female vocals, as there's a huge presence to singers like Jewel, Eva Cassidy, and Liz Fraser (and other higher-pitched voices). It can resolve detail in their voices to an unmatched level - whispers, throat-catching, off-key mistakes, etc. The second thing it does extremely well, is provide a surround soundstage. Instruments and voices that are obviously placed in the foreground during the mastering process are really brought out, while the rest of the mix gets a background "displacement" of sorts. A very 3D presentation overall. It seems to have a kind of intrinsic "crossfeed" type behavior that helps with this. The bad? Well it boils down to the following: bass too lean for its own good, highs that lack power, decay that's too fast, and layer separation that peels them apart too much so it's distracting. It's unknown why the bass still hasn't bloomed. There is some extension to it, but it lacks FORCE. The highs lacking power - well there's enough clarity to parts like cymbals and strings, but they just sound weak, and like they're in the background, even if the recording places them more forward. Violin solos do not take forward center stage when intended (tested against "The Village [OST]" for this). Attack is nice and fast, on par with the HD600, but there's almost no decay. Percussion instruments especially suffer without their natural acoustic reverb, and cymbal crashes/swishes stop early. The layer separation - well you'll either like it or dislike it. It's a veritable "peeling away the layers so they're all in their separate space" presentation. Because of this, the K701 just doesn't give a nice meshed mix to the music. Finally, there seems to be a spike somewhere in the upper mids - if the frequencies were there on the recording, it sounded honky. This was especially evident on Radiohead's "No Surprises."
So in the end, I discovered a new favorite in the HD600 alongside the DT880. They're similar enough in that they're both relatively neutral, but the DT800 resolves more detail, particularly in the highs. The HD600 seems to have its own resolution abilities in the mids though, allowing for a great deal of transparency. To put the attack/decay ratios of the three in perspective, they seemed to be about: 90%/10% (K701), 75%/25% (HD600), 40%/60% (DT880).
On a final note, the Signal Cable I got was a significant upgrade. It replaced an Ixos double-corded design. Made of the same material as the Blue Dragon. Overall resolution was just finer, with more sounds coming from the highs and mids. Bass also seemed more extended with it. I definitely would not have been able to pick apart as many differences between the headphones without it.