It's partly my fault, I started messing with the earpads because I didn't like how my HE500 sounded, and so all the modification steps are actually HE500-specific (as it is quite apparent in the measurements, how everything lines up); I spent maybe 1/20th the time testing the individual steps and their effects on sound with HE400, more of an after-thought than anything.
At any rate, I still quite dig the open vents for HE400, again, the open vents' main addition to the sound is not anything bass-related (that's only an artifact), but rather they make the imaging project more openly width-wise. With the vents closed, they are nothing more than slight angling for the pads. At least for my preference, I think the messiness it leaves with HE400 below 50 Hz is far less significant than the more open imaging/staging it provides.
For me, it's just a matter of the kind of music I'm listening to with the HE-400: EDM. The roll-off is somewhat troublesome but more importantly the adjacent mids sound significantly recessed to me. When listening to tunes with a lot of bass quantity, the mids definitely suffer. Also, given the genre choice, I can't really comment on the airiness with the vent. Again though, the issues I have with the vent can be more easily subdued by changing its size. I bet there is a certain vent size at which all the bass is nearly gone but I'm guessing there is a "sweet spot" where you can obtain the largest 50Hz bump (I'm thinking of this as a quadratic equation -- could be wrong though).