Springs under turntable

I picked up a set of springs for $35 on Amazon. I intended to use them under a preamp but one thing led to another and I tried them under the turntable. Now, this is no mean feat. It’s a Garrard 401 in a 60pound 50mm slate plinth. The spring device is interesting. It’s sold under the Nobsound brand and is made up of two 45mm wide solid billets of aluminum endcaps with recesses to fit up to seven small springs. It’s very well made. You can add or remove springs depending on the weight distribution. I had to do this with a level and it only took a few minutes. They look good. I did not fit them for floor isolation as I have concrete. I played a few tracks before fitting, and played the same tracks after fitting. Improvement in bass definition, speed, air, inner detail, more space around instruments, nicer timbre and color. Pleasant surprise for little money.
I made a big mistake. It is not a resonance frequency I was talking about above but the energy or power required to move the concrete. If you could pluck the cables holding up those concrete slabs the frequency would be very high because the cables are stretched tight and they are not very long. In buildings I believe the concrete is going to be placed on steel girders. The resonance frequency of the floor would be determined by the weight of the floors and the flex of the steel girders. The only thing I could hear when I lived in a 19 story apartment building was a low frequency boom when someone upstarts jumped down on the floor. A person weighting around 150 lb is going to put way more energy into the floor than any HiFi could. I had RH Labs subwoofers at the time and the only complainers were my next door neighbors. Nobody up or down ever complained.
Uberwaltz, you measure it!  Check out this https://www.parts-express.com/dayton-audio-omnimic-v2-acoustic-measurement-system--390-792. My room control system essentially does the same thing. It measures each individual speaker. The frequency response above 10 kHz is significantly different in my right ESL than in the Left one. The only difference is the right one has a window in front of it on the side wall. The system corrected it but beforehand the window was smearing the image.
To produce the best image the frequency response of both speakers has to be identical or the image will smear to the side that is louder at any given frequency. There are always differences from one speaker to another of the same type not to mention that the speakers occupy different positions in the room. If you measure your system you will be amazed if not terrified at the variations in frequency response particularly below 100 Hz. The curve will look like a profile of the alps. 
It is a lot of fun to measure other people's system and show them how bad things are. 

So, Uberwaltz, do not downplay those windows. More than likely they are having a significant effect on the sound. But, it is hard to predict how. If you want to know measure it. But if you are not planning to get a room control unit don't bother. Ignorance is bliss. 
I KNOW the windows are having a huge effect, hence the semi treatments to all of them.
Wood blinds on all of the side and back windows usually at 2/3 closed position.
Recent addition of heavy blackout curtains behind my gear and speakers on the large glass doors that open into the room, that had an even bigger impact than I expected.
But no I have no intention of getting any type of room control unit for this room atm.
I can live with ignorance is bliss because right now my listening experience is pure bliss!
Could it be better?
I am positive it can but small steps.
Every electronically-based room equalizer that I have ever heard does far more damage to the sound than it does good for the sound. So I agree with your strategy of altering the room, rather than the signal to give you your desired response at your listening seat.
Traffic though the tunnel created very low frequency rumble exciting the panels cracking them.
The failure was in the epoxy holding the panels. They didn't crack due to rumble.