Pros: Clear, detailed, but unoffensive treble. Detailed and slightly warm treble. Nice, tight bass. Pleasant and laid-back presentation.
Cons: None. Unless you count bad isolation - but that can't be asked for in clip-ons anyway.
Another elusive Audio-Technica model that was never sold in the States - I picked these up in Taiwan during a visit back in 2008 (I think - a bit fuzzy on the year). Production had already ceased for this model by the time I bought it, with the new ATH-EMx00 series replacing the old product line. I seem to have a knack for buying headphones that are freshly discontinued - my ATH-A9X was discontinued the year after I bought it as well.
Comparison to ATH-EM700
Interestingly, even though the ATH-EM9d was being phased out and replaced by the ATH-EM700, the older model remained more expensive than the new (the EM9d was around $190, while the EM700 was $150 brand new). I asked the shop owner (who was 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 everywhere."
He claimed the the new cord uses lower grade copper - I couldn't tell, of course, but I did notice that the kind of plastic used on the EM700's cord is indeed the cheaper kind (harder, more shiny, tangles easily), while the kind of plastic used by the EM9d's cord is the same one used on the EW9 (soft to the touch, remains straight and doesn't tangle after being uncoiled). The craftsmanship also looks to be of significantly better quality on the EM9d.
ATH-EM700 VS ATH-EM9d
Sound quality-wise, the EM9d trashed the EM700. There's more bass, the mid-range is more neutral and more defined, and the treble has a lot more clarity and detail. The EM700 sounded very muddy in comparison.
Construction Quality & Presentation
The packaging of the EM9d is the same as the rest of the Audio-Technica's clip-on line - very understated but elegant and classy. I like it.
Like the other clip-on models, the EM9d comes with a carrying pouch and an extension cord. While neither of those are befitting of the ATH-EW9 in my opinion, they complement the EM9d perfectly.
As you can clearly see, the EM9d are some very sharp-looking headphones. Audio-Technica did not skimp on the craftsmanship for these little guys. Their construction quality is on-par with the EW9. It uses the same kind of cord as well as the same kind of earclips (they changed earclip design on the EM700, and I personally didn't find them as comfortable, because the rubber part is fixed and can't wedge out to conform to the ears).
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 ATH-EW9 was popularly dubbed the "Queen of Female Vocal" in the clip-on arena, which was an appealing trait for me. The EM9d, on the other hand, was described as being the most neutral and balanced clip-on.
Owning both (I went with the EM9d for myself, but ended up buying EW9 for my wife 3 years later), I can confirm that the nicknames are well-deserved. The EM9d's sound presentation is very slightly warm, and fairly laid back. Everything is well-balanced (nothing bleeds into anything else), and sounds very natural. They are just pleasant to listen to.
It's worth mentioning that the drivers on the EM9d are titanium-plated. I had worried this would make them sound sharp and metallic (I owned an old pair of full-sized Koss that had titanium plating, and they were exactly that). Fortunately, this was not that case at all.
Treble: The treble has very good clarity but is not overly bright on the EM9d, which is great for me (because I hate sibilance). Everything that should be there is there - but the details are presented in a non-offensive manner. This was surprising, again, considering the titanium-plated drivers.
Mid-Range: I had feared that "neutral and balanced" meant the EM9d would sound cold and analytical, but this wasn't the case at all. Mid-range is not recessed - but not very pronounced either. I would say it's slightly warm, but not nearly as warm as my ATH-A9X, and much, much less warm than the EW9. Overall mids have good body and volume, without drowning out the treble (which is an issue for me on the EW9). Very naturally-balanced.
Bass: For clip-ons, the EM9d actually packs quite a bit of punch. The quality of the bass is good - nice and tight - quantity of the bass is similar to the EW9, perhaps just a tad stronger. Once again, not for bass-heads - but achieving bass-head level output wouldn't really be possible on unsealed clip-ons.
Sound Stage: Sound stage is decently large, being non-sealed. Positioning is good.
Instrument Separation: Very good, thanks to detailed treble and mid-range. Orchestral pieces are wonderfully layered and pleasant.
Isolation: Oh why am I even bothering. They are okay in quiet settings....
If for any reason you want to invest in a good pair of clip-ons, you should try very hard to find yourself a pair of ATH-EM9d. They are just so natural and pleasant to listen to, and the slightly warm presentation makes them "musical" rather than "analytical" - just a great all-arounder. To me, these sound better than the ATH-EW9 - which is too specialized and unbalanced for my taste. Don't bother with the ATH-EM700 either - the EM9d will crush them.