Dustcover Blues

You can see that up against the side wall the bass is way up

My surmise is that is your room boundary effect and may also be complicated by a standing wave (looks like you have cancellation in the center of the room). A distributed bass array might be really helpful. You'd only need a pair of subs to break up the standing wave. IIRC sounds like you already have them. If they are located by the Sound Labs  they are probably exacerbating the problem- they need to be elsewhere in the room.

Once the bass is evenly distributed you may not have this problem anymore.

I never had a dust cover for my Cosmos (it was serial number 0).

I suspect your alcove is a bass node. Have you tried putting the 'table in a different spot?

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.

Works for me Lew. Obviously, I am having trouble involving the dust cover. It is a first for me and my music habits have not changed. Once I fix the problem the dust cover will be all benefit again:-)

@atmasphere , Actually, because there is no rear wall and I use dipole linear arrays my room is very well behaved. The turntable is adjacent to the listening position. The bass is due to the boundary effect. This happens in almost every room. The table handles this just fine with the dust cover open. I am not putting it in the middle of the room.

I already have four subwoofers soon to be 8. Although they meet the definition of a "swarm" system they are arrayed in a specific pattern for a reason. At any rate the problem is not with the subs. The problem is a resonating turntable and it only happens with a few records cranked to the max. Next weekend I will make that skirt to see if closing off the space under the turntable will stop the resonance. The dustcover is now effectively sealed. Sealing off the bottom will prevent air pressure from having immediate access to the interior of the table.

The bass is due to the boundary effect.

Yes. But it appears you have a standing wave; if you were to break that up the boundary effect might be a bit reduced (that's how it worked out in my room anyway...). From how you've written about this, it appears that if you make just a bit of an improvement the problem would be gone since it only happens with certain cuts at higher volumes. I forgot to ask- what sort of stand are you using for the turntable?

Look at my system page and you can see where the turntable is. It is on a granite slab sitting on a built in walnt equipment/record cabinet (which I made) which rests on a concrete slab. The uprights are 1 7/16" plywood. All the shelves are braced. The entire affair sits in an alcove in the side wall. 

On my system page you can also see graphs of the frequency response at the turntable dustcover up and down. There is a definite rise in the bass but no really big peak or trough. Over 1 kHz the response drops 10 dB with the cover down.