I was longtime owner and user of the HD-25. Recently I acquired the Sennheiser Amperior which has the sound of the HD-25 with a bit more bass to my memory. It's very good, but I can see why Sennheiser may have pulled it off the shelf in favour of pushing the Momentum as their consumer line.
For testing, I'm running direct from iPhone 4s to Native Instruments Traktor Audio 2. It's not a big difference from going just out of the phone on its own. Using mostly 16/24 bit files, flacs, and some SACD rips. The DAC I have tops out at 48khz and 24bit output. All of these headphones are designed for use out of a portable. The Denon seems to benefit the most from a little amplification. I'd be curious to hear the all of these headphones out of better components but I have feeling the comparison below would still hold true.
The D600 is quite comfy. The ear pad shape helps and the whole headphone fits like a glove on my somewhat large head. I've heard of fit issues with smaller heads and I can see this being an issue as the fit of the D600 seems to start at large head size and go to massive bobble head. The headphone has some heft. It's not as comfortable or as light as say an HD598. It's good though, and I've worn them for hours at a time. You could watch an entire movie and you'd be good.
Amperiors, well the good stuff is the velour pads are nice to the touch. They are on-ear so they need to feel good, but there is a bit of pressure. I can see is causing problems for some. It gives you a nice seal but the tension could ruin it for some. It's the kind of headphone that's extremely easy to take off and on but for extended listening the D600 is better. I notice myself just adjusting them once and while to reduce pressure and it works.
EX600 it's an IEM, but it fits so snug in the ear. Combine that with lightness and arguably they are the most comfortable of all three.
Sound quality / presentation:
The D600 - really feeling like these headphones have not got their due. There are many fans of the HD-25 (myself included), yet had someone came up to me years ago and said "hey, how would you like your HD-25s to be less fatiguing, have deeper bass, larger soundstage and keep the detail?" I would have jumped to it. Well that in my experience is the D600. The D600 has a deep, laid back tonal signature but still has the detail. The only problem is that it is not as accurate. Snares that are sharp on the Amperior are rolled off on the D600.
Then there is the bass, which is just yummy. Many headphones just seem to lack in this area, but the D600 has dimension to its sub bass. Be it dub, or listening to acoustic bass in a jazz ensemble, you'll appreciate that it's there (though perhaps too upfront). It's the kind of headphone where if you want to really push up the volume so you can literally feel the bass, the rest of the frequency spectrum isn't going to hurt.
Amperior - I'd say both the Denon and the Amperior have comparable high end. The tonal balance is right in the middle of the other headphones. It's a bit dry sounding. Less spacious. It can be still very pleasant, and listening to Bill Evans 'Nardis' all the instruments have fine balance, whereas the same track on the Denon the bass was over present. The bass on the Amperior just does not go as deep, or is more rolled off. It is still punchy and of good quality. An example of this is listening to the drum n bass album 'Etherwood' by Etherwood. The song 'Hold Your Breath' just touches down on the D600 with it presentation of the main bass line. The Amperior, prances a bit with this track, with the snares being prominent, and not having the deep low end it needs to fully convey the piece. Though for most electronic music styles the Amperior is still quite good. Plenty of 'slam'.
MDR-EX600. Switching over using the same piece from Bill Evans - wham! You are up at front row, and there is clarity and excellent mid-range presentation. There is a natural, vivid timbre to be found here. This dramatically affects the listening experience and the emotional involvement. It is certainly more intimate, and being an in-ear also has that effect. Something else I noticed in this comparison, was the noise floor of recordings was much more noticeable in the EX600, followed by the Amperior and then the Denon. Piano were best on the EX600, with the Denon still doing great in this respect, but sounding a bit flat on the Amperior. The bass is a bit rolled off but extends quite deep with the EX600. These are hard to fault, and I'd have to say the sound quality in some respects is above that of the Amperior, and it's a more natural presentation then the D600.
In summary, I'll need a bit more time but I'm really enjoying the Amperior's balance, where it has enough bass to not be overpowering, and is more laid back then the EX600. The D600 can be a great listening experience, it has more of a sense of space than the others, more depth, but a lot of this has to do with how the headphone is tuned. It is this signature that can be a bit artificial, and can be fatiguing in its own right. You can sink into it for a while and putting on one of the other headphones after will make you feel like you came up for air. Of course, the reverse can happen, from going from the Amperior to the D600 where everything just sound so much fuller, with deep rich bass.
The Amperior in my mind is still a tool, like its elder the HD-25 it was built for task work be it ENG, djing, and tracking. It's more like listening to studio monitors that have a bit of low end boost. It is thinner sounding but it has a great balance going for it. The EX600 is awesome - micro detail, great low end and very good mids. The Ex600 is airy but still has body.
It would great to have a headphone with the low end of the D600, the natural mids of the EX600, and the balance of the Amperior.
Some other notes: the D600 comes with two cables, one lengthy, thick, high quality one and the other for mobile. Both sound good, but the long one is enough to stretch across a small room which is great for movies or gaming. The Amperior, no carrying bag included, and what is with the skimpy cables? While the HD-25 may still be more suited to DJ work the Amperior still has the swivel side cup, which is great as I DJ regularly. The EX600 is well built, comes with many tips, and two cables.
Update: check my sig to see what I eventually kept.
update Sept 2014: see below. I sold the EX600. Then sold the Amperior. Still have the D600 but have added an MDR-7550.
Also, finally got me HD25 up and running and it is a totally different headphone than the Amperior as far as sound goes. Here is link to a thread where I post me impressions http://www.head-fi.org/t/704613/review-sennheiser-hd25-1-ii-vs-sennheiser-amperior-comparison-review
Edited by Malfunkt - 9/9/14 at 9:51pm