Yeah, absolutely Sennheiser made a big error. However, they also created one of the absolutely best dynamic headphones ever created. So should we be thankful or disgrateful?
I look at it as follows:
1) It's a pretty benign flaw, that it can be fixed with 15 USD worth of material.
2) It's much easier to remove ringing due to chamber acoustics, than to add something that isn't there in the first place. E.g. create soundstage/imaging in headphones that are lacking, e.g. Audez'e's. Or create bass texture in the overly-distorted HD700. Etc. Etc.
However I do not want to start a flame war.
Objective evidence exists regarding the chamber acoustics of the HD800 (search for Analixus mod, arnaud etc. for lots and lots of info or check: http://www.innerfidelity.com/content/diy-modification-sennheiser-hd-800-anaxilus-mod) and subjective evidence exists of both ringing and not hearing ringing. It is to my knowledge not possible to remove ringing due to chamber acoustics without either changing the chamber or changing the driver. Neither has happened.
Those that hear none, are blessed. More music and less hard feelings. Those that hear it, well, there's a quick fix.
To my knowledge I haven't discussed wether it's a 3 kHz peak or a 6 kHz peak. Don't know why you're quoting me on that! :)
All the best,