The attached graph shoes measurements taken by DMS of the Diana V2.
As you can see, the difference when sealed (RED) vs unsealed (Yellow) is quite apparent. (Note: The red is not a 'forced' seal, it is just with the headphone resting naturally on the fixture)
DMS Said: "if you force the seal you can force it to be totally flat in the bass more than this, but ofc that doesnt heppen in real life".
So its a question of two extremes, yes, a forced seal can give excessive bass but is unrealistic. And you shouldn't measure it that way. Meanwhile a broken seal from a flatplate will go the other way, and will result in almost no bass, as well as significant resonance around 80hz. And so they also shouldn't be tested in that manner.
View attachment 105724
Below is the THD measurement for the diana V2
View attachment 105725
And then below is the THD measurement with the seal intentionally broken.
View attachment 105726
This shows a clear difference. I am not at all saying anything in regards to who's measurements are more trustworthy than someone else's. The way I see it the more measurements are available the better. No measurement is going to show an "absolute truth" and the more information is available the better.
But this is not a question of manufacturer integrity with their measurements, or performance of a measurement rig, this is about a clear issue with the specific method of testing these headphones, and a failure to mention this in the review.
It would be unfair not to point this out to those looking at these headphones or competitors to it. If I were spending this much, I would want to know how the headphone will perform, on my head, as intended.
If you believe these measurements are not trustworthy, then they can be easily disproven either by measuring again with a correct seal, or by measuring some other planar headphone with the seal broken and showing that it doesn't have the same effect.