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These are a great set of - sadly, discontinued - closed headphones that can give today's best ones a real run for their money.

Originally developed as high-end consumer headphones but studios soon adopted them for their detail-oriented and relatively accurate sound.
The construction is pretty simple but quite durable. The cable is replaceable as well, I had no trouble fitting the aftermarket HD600/HD650 cables.
The comfort is excellent thanks to the light weight and snug over-ear fit with little clamping force. And yet these still isolate much better than most circumaural headphones, if not as good as the HD25s. So they are not unusable on the go but clearly not designed for that purpose, they are a bit bulky.

The sound is fairly balanced with a slightly V-shaped sound signature which makes them suitable for lower level listening but not for the people who like the midrange to stand out this is more about hearing the whole performance. The cause of the slight dip in the mid is diffuse field equalization, a skewed frequency balance developed to make the sound presentation more like that created by loudspeakers.
The result is the best soundstage I heard from a closed headphone with isolation, when aided by and airy powerful amp it is outstanding - but it is not going to rival the best open headphones and sometimes it might feel a bit diffused but I enjoy the out of the head sounds I get sometimes.
Another highlight is the bass which is goes amazingly deep, 20 Hz is clearly audible, if you love deep bass you will love these (great for movies as well). However, the mid-bass is slightly boomy but still quite good compared to other closed rivals.
As far as treble is concerned it is also very good, very extended and weighty not veiled (like the HD6xx series which are also a bit slower) although quite revealing so it is a better pairing with a smooth source otherwise it can be grainy.

They are not the most efficient so it is highly recommended to drive them with a powerful (preferably warm, musical and punchy) amp to give better control of the bass which also results in increased clarity, balanced drive is also welcome.

Overall just a great set of headphones which should still be produced today (or in further developed form) and if you start to look for more information about them you will find other users wishing the same as well.

After speding some time with other top-tier headphones it is the midrange that seem to be the main weak point but based on how big a simple headphone cable change can bring at a certain level, I might just find one that can help with this issue. The people who like how the HD25 sounds will probably like these as well and it fits to the family of the Sennheiser headphones that have a laid back midrange.