Pros: Excellent tonal balance and extension. Very natural timbre. Superb construction, materials, and comfort level.
Cons: Poor isolation, light on the bass.
The ATH-EM9d is an elusive model that was never sold in the United States - I picked these up in Taiwan during a trip to Taiwan in 2008. Production had already ceased at the time of purchase, with the new ATH-EMx00 series replacing the old line.
Comparison to ATH-EM700
Even though the ATH-EM9d was being replaced by the ATH-EM700, the older model remained more expensive than the new one (the old EM9d was $190, while the EM700 was $150 brand new). I asked the shop owner (who is a long-time audiophile himself, and participated actively on several headphone forums) what he thought, and he said quite simply: "The new ones are junk. AT cut corners in materials all over the place."
One thing that was easily noticeable was the difference in the plastic used on the cords. The EM9d has the same kind of high-quality tangle-free plastic used on expensive headphones, while the kind used on the EM700 feels like the ones used on budget earbuds. There's also a lot more metal on the housing of the EM9d, whereas the EM700 uses a lot more plastic. I would say the older model does indeed seem much higher quality in both the materials and construction.
ATH-EM700 (left) & ATH-EM9d (right)
I did some brief listening of both, and the conclusion was easily arrived at... The EM9d trashed the EM700. The EM9d has much better extension and clarity, and the tonality is much more balanced. The EM700 was mostly mid-centric with relatively poor extension and sounded very muddy in comparison.
Materials & Craftsmanship
The packaging of the EM9d is the same as the rest of the Audio-Technica's clip-on line - very understated but classy. They come with a carrying pouch and an extension cord.
As you can clearly see, the EM9d are very sharp-looking headphones. Construction quality is on-par with the EW9. It uses the same grade of cord and the same earclips (which were redesigned on the EM700, and were not as comfortable). Comfort level is nothing short of excellent - I've worn these to sleep many times; you simply forget they are even there.
The earclip design of the EM9d. Note the flexible rubber cushion and the height adjustment.
Audio-Technica is the only headphone manufacturer that makes high-end clip-on type headphones, so I read up on both the ATH-EW9 and the ATH-EM9d extensively before I went down to the shop. The EW9 is popularly referred to as the "Queen of Female Vocals" for clip-ons in the Taiwanese audiophile circles, while the EM9d reportedly was the "King of Neutrality".
I now own both the EW9 and the EM9d, and can confirm that the reviews I read were accurate. The EM9d is indeed very neutral; tonal balance is nothing short of excellent. The EW9, while indeed great for vocals with its thick, sweet mid-range, is only suitable for some types of music and lends too much coloration to the music for my taste.
It's worth mentioning that the drivers on the EM9d are titanium-plated, which likely contributes to its excellent clarity and treble performance.
Also note that it's a fully open-backed headphone (you can actually see the driver through the holes on the back of the housing). As such, the EM9d offers absolutely no isolation - which makes it suitable for certain situations where awareness of one's surrounding is desired (such as walking around on foot), but not so much for others (riding in a noisy subway train).
Treble: The treble on the EM9d offers excellent clarity and extends very well, but is otherwise quite smooth and unoffensive. It's not very sparkly and it's not for trebleheads, but I personally really like it. Sibilance is never an issue with these.
Mid-Range: The mid-range is very clear and just slightly north of neutral, but still in balanced territory. This is not the signature AT house sound, but IMO it's for the best. Instruments have very natural timbre, and listening to classical pieces on these will put a smile on your face.
Bass: For fully open-backed clip-ons, the bass on the EM9d is actually quite ok. It extends decently well and packs just enough punch to lend classical instruments proper heft (and the bass is very controlled and tight). Quantitatively, though, it is definitely on the light side and will not play well with bass-heavy genres.
Sound Stage: The sound stage on the EM9d is very open and airy, which is not surprising given its open-backed nature. The out-of-head feel of the sound and the lack of isolation often lends the strange sensation that you are carrying around an invisible stereo system while walking about.
If for any reason you want to invest in a good pair of clip-ons, you should try hard to find yourself a pair of ATH-EM9d. These are my absolute favorite in the category. For $190 though, there are better-sounding headphones outside of the clip-on category to be had - many good IEM's or over-the-ear portables are in this price range or below. So I am knocking this one down in the value department a bit.