100+ Head-Fier
Pros: Intoxicatingly sweet mid-range, nicely backed by a full and well-controlled upper/mid bass and a non-intrusive treble.
Cons: Rolled off treble and bass. Small sound stage. Not very resolving compared to modern cans.
I am the fortunate owner of a fairly rare pair of headphone: The Audio Technica ATH-A9X. It is the precursor to the highly popular and successful ATH-A900, and I feel they deserve a proper review (since so few people own them outside of Asia).
The Audio-Technica ATH-A9X was part of the second-generation "Art Monitor" headphone series that surfaced during the late-90's (it was produced between 1997-2002), alongside its siblings the A5X and A7X. It was preceded by the first-generation ATH-A9 (introduced in 1994) and succeded by the ATH-A900.  I do not own a ATH-A900, though I do own the the newest incarnation in the series - the ATH-A900X, and I'll be happy to do a brief comparison to show you how this popular headphone series has progressed and how it has stayed the same.  
Build Quality & Comfort
The old ATH-AXX series was never sold in the United States. I purchased it during a trip to Taiwan in 2001. It is a very unique-looking and eye-catching headphone, and one would probably love it or hate it. That said, it is undeniable that the material and construction quality are very high - in fact I find it superior to both the ATH-A900 and the A900X in this regard.
As you can see, the cups are made of ultra-polished aluminum alloy. They look very slick, though fingerprints stick very easily.
The earpads on the ATH-A9X were very high-quality protein leather. It was soft, supple, and non-sticky (it breathes better than the kind used on the AKG K550), which made me really sad when I saw the kind of cheap pleather Audio-Technica chose to ship the ATH-A900 with. The A900X uses better-quality pleather than the A900, but still a farcry from what the A9X had. 
Like many of the enthusiast-grade Audio-Technica headphones, the ATH-A9X came with the famous "wing-support" system, and sits very comfortably on the head. This is in spite of their weight - at 350g, the A9X is pretty hefty for a dynamic headphone. However, the A9X has superior clamping force and the wings exert better downward pressure than the A900 and A900X (whose wings feel flimsy in comparison). They do not slip down and press against the top of the ears as its successors do. 
Several other traits of the A9X that are worth noting:
  1. The plastic parts on the A9X are not monotone, but an elegant "metallic" brown (it has shiny metallic flakes in the material). It's subtle, but very elegant and tastefully implemented.
  2. The cord is wrapped in tangle-free fabric, which is a silky brown color that complements the plastic frame. It looks a lot better than the simple black cords on the A900 and A900X, in my opinion.
  3. The padding on the "wings" are also encased in high-quality pleather, as opposed to the simple fabric covers on the A900 and A900X.  This is a small but very nice stylistic touch in my opinion, as the wings complement the earpads very well when the headphones are not in use, and the whole thing just looks more elegant in comparison to its younger cousins.
Sound Quality
The A9X has what - to my understanding - is called the "old school" Audio Technica sound. Intoxicatingly thick and warm in the mids, with rolled-off treble and bass. Here are its technical specifications, as provided by Audio Technica's Japanese website.
Technical Specification:
  1. Mirror-polished, high-strength aluminum alloy housing.
  2. Special coating completely absorbs unwanted vibrations and provides superior isolation.
  3. Employs same φ53mm high-quality drivers from flagship model A10ANV.
  4. Frequency Response: 5 - 30,000Hz
  5. Sensitivity: 103dB/mW
  6. Maximum Output: 2,000mW
  7. P.A.T. anti-resonance mechanism.
  8. High-quality cloth-wrapped PCOCC signal cord.
  9. Specially-formulated pleather earpads.
*According to the specification sheet, the aluminum housing, the A10ANV-class drivers, the PCOCC signal cord, and the "special" pleather earpads are all unique to the A9X (not shared by the A7X and A5X).
Treble: The treble presentation on the ATH-A9X is rolled off and laid-back. The younger A900X is noticeably sparklier and more resolving, but depending on your preferences they can be more fatiguing. The A9X is very good for relaxed listening.
Mid-Range: The mid-range is A9X's strong suite (if you like the AT house sound). It is warm, thick, and forward. Vocals sound very sweet and full-bodied, and instruments (particularly wind and string) are intimate and inviting. I find the A9X to be excellent for smaller, more intimate pieces (small or solo performances). It lacks the resolution and expansive sound stage of its younger cousin to properly render complex orchestral pieces. The A900X, in comparison, sounds significantly cleaner and more neutral.
Bass: The A9X has an adequate amount of upper and mid bass to give proper presence to most instruments, and when present, it is decently controlled. However, the bass is rolled off quite early and there isn't much sub-bass that I can hear. Quantitatively the A9X is on the bass-light side, and isn't suitable for modern genres.
Sound Stage: The A9X has a curiously small soundstage (possibly the smallest I've ever heard in a headphone) - singers literally sound like they are signing into your ears. This works well for some pieces but not for others. It's an interesting quality and I wonder why AT tuned them this way. If I have one big gripe about the A9X, this would be it.
The ATH-A9X is a unique-looking headphone with a sound signature that old school Audio Technica fans enjoyed. Its younger cousins - the A900 and A900X have been progressing more and more toward being tonally balanced and neutral, so the likes of the A9X will likely fade away. That said, it is a small piece in AT's history that I will gladly hold onto.
5 - 30,000MHz ??
Thanks for sharing. Never knew about this model.