Dustcover Blues

@tvad , Yes, you are quite correct. The very best tables came with dust covers and for a time even the less expensive tables came with them. Then manufacturers learned from Transcriptors (I think) that if you made your turntables cool looking enough you could sell a lot of them even if they are lousy tables. I should know as I fell for that. Soon they became so odd shaped there was no way you could attach a dust cover. But, visual stimuli dramatically improves the hearing of a large population of people. This whole mess ends at the Techdas Zero, without a question the coolest looking turntable on the planet and technically it is very impressive. Personally, I would rather spend that money rescuing geto kids from a toxic environent and getting them someplace where kids can have fun being kids like a summer camp. 

put the dust cover in the recyclable basket for pickup

Leaving aside our differences regarding to use or not to use a dust cover, I am thinking about your hypothesis that right now your turntable/cover are together acting according to the principle of a Helmholtz Resonator.  The idea does "resonate", but so far as I can tell from my reading, a Helmholtz Resonator requires air (or whatever fluid is resonating) to be put into motion.  Energy has to be put into the system for that to happen.  In your conception, what is the source of energy?  I would offer an alternative hypothesis, for good or ill. Could it be that energy being emitted by the cartridge body, the same energy that is heard as "cartridge noise" is bouncing around under the cover which is by chance tuned to resonate at a low bass frequency, which is why it is excited only when you play certain LPs with low bass frequencies and at a certain SPL above some threshold that causes the whole thing to get excited and re-enforce itself via positive feedback?  That is what I meant when I remarked that you are experiencing a problem that I associate with dust covers per se.  I put this out there for discussion, not to antagonize or criticize. In my own experience, when I played LPs with dust cover down, inevitably the dust cover itself could be felt to be vibrating, I always thought sympathetically with cartridge vibrations.

The dust cover is there to prevent dust build up on the equipment, not the record. Remove it when playing a record.  

Curious problem. I hear no difference in sound whether the dust cover is up or down on my TD 166 mk2 Thoreans.