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suppose it's all there to tune the sound but you gotta remember that whenever you tune sound using foam and membranes you also take away alot of details, directness and ease in the music. In the extreme case scenario imagine listening to a pair of headphones through a pillow, how good does that sound? Ideally you want as little as possible between your ears and the headphone drivers. The more direct path the better, the rest is EQ if needed.
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I came in here fearing that you would blurt the opposite of all that.
But yeah, when tweaking the sound with physical modifications, sticking stuff between the ears and the drivers should be avoided in general, something some fellow modders don't quite get around here.
Treble harshness is a result of the drivers' vibration energy radiating into the surrounding material then that material returning the vibrations to the driver (or so my pet theory goes, ha), and Dynamat Xtreme is my go-to dampening material to alleviate this situation.
Well whatever is going on, Dynamat brings welcome improvement. Depending on where you place it, it can improve bass response, soundstaging, eliminate treble harshmess (and sometimes even some treble peaks), improve transparency, eliminate grain, and more depending on the headphone.
If you only listen to classical or midrange focused music, and have no problems with open cans, I think you should still listen to and consider the HD600/650, and probably the new Hifiman 400S as well. But if you specifically want a closed set of cans with a neutral reference type sound, and which can play a bit of everything, these are you babies right here.
"Neutral reference" as you call it here IS the best for classical.
Basses have "sonic information" in the midrange and treble, especially midrange. Without good bass, the "foundation" of classical music will be lacking, which is not how it is meant to work, not how it is meant to be performed live. Treble affects all, even imaging to an extent, and good soundstaging is preferred, so good treble is preferable. Then the midrange cannot sound its very best without good treble, and to a lesser extent, good bass.
That said, there are plenty who consider HD600 their "neutral reference" headphone. I don't even begin to agree, but that's just me. I do not find HD600 neutral, it definitely has more of a flavor of its own than what would be "ground truth" of sound.
On a related note, HD6x0 have a pronounced "organic" quality to the sound that makes acoustic instrument timbres come through with a higher degree of "believability" or something that is uncommon among headphones. Heck, this is part of the Sennheiser house sound in general, in my experience, as you can also hear this in HD25, HD559, PX200, HD205, and so forth, to varying degrees. SRH840 and the Hifiman RE0 are a couple of "organic-sounding" outliers, akin to Sennheisers, for example. This sonic quality helps boost HD6x0's reputation as a classical can.
Lastly, I am so loving the sound of all these Tesla Beyers that have been coming out. Can't wait to score a DT1990, or I may just go with 1770 instead.