there is something about sennheiser that makes it special. it crosses all of their high end cans, from the HD580 to the mighty orpheus... it actually has to do with how the strings of an orchestra sound. i believe that this is the "tonality" that daverose speaks about and i can completely agree with.
nevertheless i agree with Humanflyz that there are limitations that are unkown until they are compared to higher end gear. the first recording was said to be a perfect rendition of live sound... yet if anyone ever heard it today, it would be called low-fi. why? because there have been changes that have raised our expectations for what sound reproduction can be like. when i go out and start listening to big K1000 rigs, the e-stats and those high end sony and ATH headphones the limitations of the HD650 become clear. it is one of those limitations, the inability to project full resolution and detail (although the HD650 does have alot of detail, just not as much) that i like to call the Sennheiser veil. this doesn't make me hate my headphone, it just lets me know that there are other things out there. (really,i'm talking in absolute terms, the HD650 is not actually lacking in detail, it just doesn't have the full 100 percent).
no, i'm not a real fan of the K1000, i think it lacks weight, sounds edgy and doesn't always play notes with authority.. nevertheless it does do some wonderful things with the soundstage and it's detail that are noteworthy when comparing them to other cans.
also i dont' always buy these "different ears" or "what is neutral/natural to you" arguements. if it is there, it is there if it's not then it's not. why... it's simple, dada, Humanflyz, yellafella321 all have different music preferences, different amps, sources and headphone likes. nevertheless, when we start comparing the same amp or source the preceptions are always the same. it's only when we get to things like "does the K1000 have enougth bass" that people start to disagree (which is explained by the fact that we all have different reference headphones).
btw, i dont' get why sennheiser has to have all these dips.. the HE90 is flat and by far it is considered to be the best can in the world.
also in my not so humble opinion: the DT990 is a piece of crap, so dont even get me started on it...