Pros: Crystal clear vocals, neutral, lightweight, cheap, LOTS of detail, amazing Imaging, good soundstage, forward mids
Cons: Slightly rolled off highs, lack of bass to some, needs amp, coiled cable, requires burn-in
I've tried about two dozen pairs of headphones and I'm always looking for the absolute best pair for vocals. I also want forward sounding mids, but not too bad. My preference is also for highs that are not harsh or fatiguing. Being somewhat rolled off is ok. I find the SRH-840 is what I like in this area.
The best thing about this headphone is the quality of vocals. It just sounds amazing with both male and female vocals. This headphone is absolutely perfect for Jpop, Cantopop, Imogen Heap and Radiohead. The strange thing about this headphone is that the sound feels like an IEM. You feel like you're right there.
The highs are somewhat rolled off, but not as badly as the HD-600, but slightly worse then the k240 (55om) and SRH-840. This and the overall sound signature makes them a VERY non fatiguing headphone in every way.
Overall, this is a very neutral souding headphone. The only exception is that the mids are slightly forward, but not too badly. About the same as the Grado SR-80 or a tad more.
Here's the strange thing. Koss labels them as having "extreme" bass. They sound like they have a lot more bass out of the box, but after burn-in it gets reduced. I don't find these to be bass light or anywhere near that. The bass is VERY well controlled and perhaps people mistake them for being bass light because of this. It has just slightly less bass then my burned in k240 on an amp. Don't buy these if you like a ton of bass. Overall I find they have about the same amount of bass as the SRH-840 or maybe slightly less.
Second best thing about these is how natural and accurate everything sounds. Basically, everything sounds pretty similar to what they sound like on my HD-600. These are extremely picky about the source quality. They're VERY revealing of lower bit-rate and poorly mastered CDs. With my HD-600 I feel they can make anything sound good.
Soundstage is very good for a closed headphone, but not great. What really impressed me is the imaging of these headphones. I basically feel as if I can pinpoint every single tiny thing. Even more impressive then my HD-600. I think it has to do with the closed design, but i'm not sure. My k240 has fairly poor imaging and that's semi-open. Instrument separation is also very good.
The problem with these headphones is that they NEED an amp. I use them with my E5 and Nuforce mobile and they don't sound as good. Despite the low ohm rating I don't suggest them without a GOOD portable amp. I use them with my Total Airhead and they sound great! With a good amp they also have a bit more bass, but not much.
Another negative is that my first pair broke due to my own fault. I bought another and they sounded bad out of the box. The mids were very recessed and they just sounded wrong. The mids are nowhere near being recessed normally. I burned them in and after the 1st night they sounded better, but not quite. On the 3rd night they were perfect and just like my last pair. My first pair didn't seem to need burn-in to sound good.
I also found out these work with the Pearstone Velour pads. They need to be stretched out though and they look a bit goofy, but not too bad. They're now extremely comfortable. Out of the box I gave the comfort a 7.5/10, but now it's a 9.5/10! Luckily they don't degrade the sound quality.
Another nice thing about these is that they're extremely small. I think they're only 6.5oz without the cable. The build quality is also excellent. The cups are 50% metal along with the internal band and one other part.
Two other headphones sound very similar in some ways. These sound almost like the Maxell DHP-II, but those are far less neutral and have more bass and mid-bass. The vocals and mids on the DJ100 are far better. The build quality on the DHP-II is also not good. I'm almost positive they share the same drivers or something close. For anyone that cares, Koss has told me that they drivers on the DJ100 are titanium. They're 40mm when I checked my pair.
The Koss Pro3aat also sounds similar, but has way too much bass for me and too much clamping force.
These headphones are not for everyone. If you focus on vocal quality and good mids, get these. If you want sparkly and very bright highs or massive amounts of bass, look elsewhere. If you love any kind of Jpop or Cantopop, these are an absolute 100% must have. I would also skip these for rap, classical or metal. Classical isn't bad on them. My k240 are far better for classical.
My 2nd favorite is the k240 Studio. Those are better for more genres, but the vocals are not as forward and clear. The overall level of detail is also just not as forward. For me, the DJ100's level of detail is a step down from the DT-880 and AD700, but not by much. The mids are like my Grado SR-80 in some ways but without the fatiguing highs (un-modded). The DJ100 has far more bass though, but less mid-bass.
So, as you couldn't tell, this is my favorite headphone i've ever owned!