In my view there is no 10dB+ variation without extraneous factors.
Yes, baffled (pun intended) me too. There seems to be a bit more going on than just seal.
I would like to get my fixture (and ears) on a pair.
Maybe visit Delft again he'll probably have one.
I hate to order one from Thomann and then return it and misuse their return policy.
If the seal behaves anything like most other planars it should behave well but I suspect some nifty trickery is going on because Dan seems to be one of the rare designers that can acoustically 'boost' the natural low bass for planar drivers and that even in a way that it follows Harman extremely closely.
Here is the seal response of 2 other (open) planars (my target, not Harman target):
Edition XS:
Avantone Pro Planar:
And a smaller sized driver Sundara (2021):
Breaking the seal usually reduces bass extension but lifts low bass.
Would be interesting to see how these DCA react to a similar seal breakage.
It should be noted that above planars are open yet might still be comparable because all of them have a closed front volume and no acoustic resistors to the rear volume.
Not even a pressure equalization hole.
I expect the DCA to have none either but could be wrong.
Anyways... nothing standard about my DIY measurements but the trend is clearly visible.
Below the closed AH-D5200 (dynamic driver) that almost does not react to seal breakage (by design):
And the mentioned AKG K371:
The ATH-M50X is more representative in bass drop-off when breaking the seal for closed headphones:
Closed HD598CS: