There is something that I think is at play in listening to music and especially A/Bing between headphones. In chord theory it is not so much the absolute frequency of the note that is important (one instance of this is the "migration" from what it once was of the absolute frequency or something like "concert" A or middle C), but it is the relationship or "distance" between notes--a 2nd or 3rd, or 5th etc. that becomes musically pleasing/displeasing. So there is the reference of where we're coming from as a sound that I think the next sound gets compared to. (This is my own pet theory, but it is based upon my experience. And somewhere in Head-fi I once saw a much better explaination of this phenomena.)
As a real world example, I listen to both the balanced Edition 9s and the Senn 650. If I quickly go from the 650s (no slouch in terms of bass) to the Edition 9s, for a little bit of time the 9s will sound off--often too much bass and sometimes too bright. Likewise if I go from listening to the Edition 9 to the 650s, then the 650s seem very muddy and the symptoms of the classic Sennheiser veil are heard. But on their own, I can enjoy each headphone very much, although generally my preference is with the Edition 9s. But as I continue listening to either hp for just a few minutes this sensation of the new one being "off" quickly abates.
Occasionally I've watched some folks put on my Edition 9s for 30 second to a minute and quickly dismiss them (I think usually not liking the bass). I don't think that this is limited to the Edition 9s or to the 650s (they've certainly had their detractors) for that matter. But it may (pure speculation on my part) show up more with the Edition 9s because few headphones, if any, reveal as much high frequency information while at the same time go as low and as full with the bass. So the Edition 9s can sound very different from two different directions so to speak.
While I don't think it accounts for everything, I believe there is some truth to the idea of "brain burn in." I suspect that this happens with all headphones and even playback sources like vinyl vs digital. So if something is really different than the signature sound someone is used to, the new sound signature can sound "off" in some way. (That's probably why most Grados except the PS-1 sound uniformly harsh to me.)
Sometimes but not always I get a sort of reverse of this on going from vinyl to digital, especially SACDs. At first the SACDs can sound cleaner and even more pleasing than the vinyl (which I generally prefer). But after listening for awhile I become progressively dissatisfied with the digital, eventually finding them colder, more sterile and artificial sounding and too "sharp."
Personally I think that there is a lot to hearing that is not fully understood. And I suspect it comes into play in a lot more than is realized in the gear comparisons we do.
And then there are the individual sound preferences....
PS. And then there are the genres of music that makes a difference. I love the Edition 9s for rock, generally don't like them as much for orchestra on vinyl (this is sometimes where I experience them as having too much bass), and love them again for female vocalists where the greater bass response translates into greater fullness and richness of the female voice.
HeadphoneAddict mentioned that he found the WA6 a little too dark and bassy with the Edition 9 - have any of you noticed the same thing? Is this upgrades/tubes dependant?
