This was demonstrated in stages of measurements for the jergpad mods. The back vents slightly rise the response around 50Hz and also noticeably increase harmonic distortion. The harmonic distortion in particular gives more of the rumble and bloom. The pad adhesive mod helps smooth this area out a bit and lowers the distortion some. Taping up the vent further flattens the bass and lowers the distortion. Entirely skipping the vents gives a flatter bass response and less harmonic distortion.
Personally, I don't mind (might actually prefer) a bit of a hump around that area, and I don't find the additional rumble and bloom to be unpleasant (might even be tastefully "fun").
Thanks for the detailed explanation. I did enjoy it, until I encountered a couple recordings where it was a negative to my listening experience.
And the vents are listed as "optional" in the guide (LOL, all modding is optional, but I get the point). Anyway, I've arrived at what I want from the HE-500, with full props to jerg and modulor.
Modding is fun