tl;dr = read the bold stuff for my bare impressions
Not trying to drag up a zombie thread, but I just noticed a PM from other allen asking if I could give my impressions about this set of cans. To be fair to him, he sent it in late 2008; to be fair to me, I have been broke until not long ago, and couldn't bear to be on head-fi watching all the audio development, knowing I couldn't partake.
First, I got mine for $1.99 at a goodwill store in town b/c no one there knew what these ugly headphones were worth (I also got a free, crappy, 1/4" headphone splitter, fools). Second, I had no idea what they were and went through a very similar process of reading everything I could, finding nothing, and then listening to whatever I could with them. Third, I have no freaking idea when they were manufactured, but I do know that their impedence is TWO THOUSAND OHMS; the little badge on the side reads "2kΩ x 2" which in Beyer nomenclature means TWO [explicit] THOUSAND.
Obvious concerns: how do you drive that beast? what can you even listen to? why the [deleted] don't you sell them to someone who could drive them, and get them off the kitchen counter, like your wife is constantly nagging you to do? Responses in order: Sometimes, when I feel like punishing my little 2move, I jam it into "high impedence" mode, and turn the knob to 11, it works, sort of; I can listen to whatever I want, thank you; I don't sell them b/c I got them SUPER cheap, and I kind of like them; also they got me into my dt150's which I absolutely love.
Seriously, though, I find them to be the thinnest, most reducing set of headphones I have ever used. Unamped they tamp everything (everything) down to its barest electrical impulse; I can hear all the notes, nothing is missing, and they are at the correct frequencies, but all I can really tell is that the note happened, nothing of what it really sounds like, even less of what the music should be. Rock song + dt100 = beat poetry recording. This makes them super fun for amateur monitoring, though to be fair, the only real world application I have found, so far, is settling bets about what the singer is saying, or what-notes-come-when in a passage of music. Amped, music sounds a little more like music, but it is still super flat; there is no "hump" anywhere (I don't mean on a response graph, I mean in my brain), and nothing is overpresent, or underpresent. Well, that isn't entirely true: everything is underpresent, these are the antithesis of fully flavored headphones like Grado et al.
That other thing I use them for: video games. When the wife is studying / doing anything productive, and I want to play some modern warfare 2, I plug these bad boys in and turn the TV volume to "ask greg for mercy." I swear I can hear EVERYTHING the game puts out, and where it is relative to me. This is useful, and makes them worth well more than the $2.10 (after tax). Also since the chasis is identical to the dt150, so I have fun from time to time swapping parts and seeing what the earpads, cables, and headbands feel like in new combination.
A brief comment about seal. These things suction to my head sufficiently that, when music is playing outside noise is even flatter than what comes out of the cans. I can see your mouth moving, but...
Anyway, hope my rambling review gave some reader a powerful reason to find these at a thrift shop, or pay me $500 for my pair.