Glad you have things started out and now you are able to "rock out" again and play your music at listening levels that only a few appreciate...I get it. I also like to play loud every once in a while. Well, perhaps, more lately;). The work you did to your system and room to ensure that you can enjoy playing music loudly without problems is what makes this hobby fun. Your subwoofer builds are intriguing to me because customized them to take advantage of using DSP, which you already use for your system. I use SVS (4 each) with the limited DSP they have, but it works.
Like you, I have lots of mass for my turntable. It is already 120 plus pounds, sitting on top of a 275 pound Clearaudio MontBlanc stand, on top of bamboo floor with anti-vibration and sound absorbing adhesive (that's what the special glue container says (275 per 5 gallon), on top of concrete. No resonance issues in this system even with 100db plus levels, which is not part of my loud norm. only once in a while.
Back to the main topic. Of course, you realized this symptom was some form of resonance emanating from the dust cover based on increasing the loudness of your music. I experienced that once with my old (still is use for living room) Denon DP62L sitting between a pair of Polk Audio SDA2's being driven by Carver M1.5Ts.
This was in the early 80's when I first joined the military and lived in the barracks, so no thought of treating room acoustics. Whenever I played Def Leppard or AC/DC really loud, I would hear that resonant feedback that you mentioned. My trick to eliminate this resonant feedback was to prop the dust cover up so that it was not entirely touching the turntable base, about a 45 to 55 degree lift. I forgot what I used, but I am sure it was some crude and rudimentary fix that only Soldiers would figure out when you wanted to make things 'happen" or accomplish the mission.
Back then I was a novice and knew virtually little about tweaking, only, plug and playing and treating issues if they were gross, bass to boomy,.move speakers away from walls, too little bass, move back to walls, not loud enough, get bigger amps, and bigger speakers, and so on and on it went for years. I have learned a lot since then...hence, my moniker, which has philosophical meaning to me, the journey will never end. We will always learn something new as we grow older or experience new things, in this hobby, other hobbies, or in life.
Hmm, now I want to dig out some Rush 2112, Three Blind Mice, and Miles Davis, talk about eclectic tastes in music, LOL. Low level listening sometimes, in the room live feeling most of the time. It is all enjoyable.