Good review - interesting in that you think the Mk.II version is better than the original 1986 version as many people thing the original was better.
In fact the Mk.I and Mk.II are identical and the only difference is in the external design of the earcup.
In fact I was involved in the design testing of the originals - back in 1986 when I was working as Technical Manager at Sennheiser UK (I am now a freelance consultant).
They were designed as closed hi-fi headphones which were quickly adopted by professionals due to their accuracy and neutrality. They were the closed equivalent to the top of the range open headphones at the time.
They were discontinued, but the replacement was not as good and the HD 250 II was brought back as professional headphones and continued in production until about 2006.
Sennheiser have not done a replacement for the HD 250, they seem to think closed = DJ and forget there are professional studios and hi-fi enthusiasts who demand a top end neutral, accurate and revealing closed headphone. Personally I have a couple of the originals and two or three pairs of the Mk.II as well as a couple of the original test units that were sent to me in 1986 (those are not very nice and the HD 250 was redesigned after I sent them the results of my tests). Personally I use the originals in preference to the Mk.II version (no - I am not selling any as they are used for musician playback when I do recording sessions).
Personally I hope Sennheiser introduce a set of closed headphones based on the ring driver technology of the HD 800 - that's what I would love.

Thanks for taking the time to write a review for the HD250, padam. I, too, have had quite a bit of listening time with both Linear I and Linear II. To begin with, I actually felt that the mkII was a slight improvement on the mkI, because of the slightly higher bass response of the mkII. However, after listening to more music material and comparing both versions over several weeks/months, I eventually came to favour the HD250 Linear I, as I felt that the bass response was more controlled than the mkII, just as extended but less resonant/overpowering with some genres of music.
 
Both are neutral, excellent headphones, which are very close to the accuracy of monitor speakers such as my LS3/5A. They both however, in my opinion, exhibit a dip in the lower midrange, as if a small section is completely cut out. I tried swapping cables and using velour pads instead of pleather, however the signature did not change.
 
I have concluded that the frequency response has intentionally been reduced, close to the resonant frequency of the enclosures to prevent significant amplitude increase?? Because of this dip, I sold all of my HD250s and I now use the HD540II and HD560II, which perfect the HD250 signature IMO.