Pros: Very flat response, nice soundstage, excellent imaging, phenomenal detail retrieval, most gorgeous headphone ever
Cons: Lack of bass and treble extension, slightly uncomfortable, hard to drive
First off I adore this headphone. It's the most beautiful headphone I own and it's also the rarest. It has a lot of things about it that make it unique, and it deserves a place in everyone's collection.
While it does not have a 'real' soundstage, it has the best imaging of any headphone I own. The sound is very directional, and seems to emanate from nowhere as opposed to two distinct places on your ears. They are very well-detailed and have fast attack. Decay could be better, but it's not bad. The fast attack and decay throughout the whole frequency range gives them an amazing sense of rhythm. They sound extremely lively and perky, and the whole sound is crisp and clear.
These headphones have a remarkably flat frequency response. No peaks, no recessions, just a totally level playing field. However, they don't have a lot of treble or bass extension. They do have bass and they do have treble, but the bass has a very steep roll-off and lacks weight and impact. This is a flaw of piezoelectric headphones; they are phyiscally incapable of reproducing bass below a certain point. The bass that is there is well textured and detailed. The treble doesn't have a lot of sparkle, but it extends much farther than the bass so I rarely notice where it rolls off.
Despite the ridiculously neutral tone and the slight analyticality of these headphones, I can't put them down. I am a moderate basshead, but they are still my favorite headphone, and my best technically. My ears adjusted to them very rapidly and I absolutely fell in love with them. Their awesome sense of rhythm engages me in the music and makes me tap my feet in pleasure.
The build quality is also phenomenal. The backs of the earcups are brushed aluminum with stiff steel mesh, the struts/yokes are brushed steel (I think), and the headband is a spring-steel core with a leather sleeve. The adjustments make a satisfying click noise when you move them, and they stay put. The cable is cloth sheathed and very long. The vinyl on my earpads has worn out and flaked off, but hey, they're 42 years old.
They do have some downfalls other than the extension issue. They are totally open, so no isolation and massive leakage. They are very difficult to drive, you absolutely need an amp, and a good one at that. They are also moderately uncomfortable as the earpads are thin and they are supra-aural. My pair might be more comfortable than most, because it is my understanding that most units clamp a lot more than mine do... I think the previous owner of mine stretched out the headband because of the comfort issue. They also sound a bit thin at times due to the stifling lack of bass; these headphones do not work with all genres for that reason.
If you find a pair of these, I would advise you to grab them. Even if you don't like their sound, they are an utterly gorgeous display piece and are the first piezoelectric headphone ever made, making them an awesome piece of history.