Yep. Audiokinesis and Duke Lejeune. Woofer array.
The question of whether the resonance has anything to do with Helmholtz is somewhat relevant, because if it were due to the Helmholtz phenomenon, then you could live on with your prior conviction about the unalloyed benefits of the dust cover. You point out that there is a pathway for air to exit the space enclosed by the sealed dust cover and plinth, downward around the platter evidently. But if that constitutes a Helmholtz Resonator, then sealing the space between the edge of the dust cover and the plinth ought to have changed the resonant frequency. Sealing the dust cover to the plinth changes both the area of the aperture and the net path length across the aperture, and those two parameters are in the equation for resonant frequency of a Helmholtz Resonator. Face it; dust covers are not all good. If you admit that, I will admit they are not all bad.