Speaker floor coupling

I have my Dynaudio S40s on their Stand 6. The stands have a thin sorbothane pad on the top between the top plate and the speaker. I have fitted the spikes, but am in a rented apartment and use metal discs under the spike to avoid floor damage. In order to reduce transmission through the floor, I have sorbothane pads under these metal disc.
Recently, I have noticed a mid bass boom on certain records. Of course, this could be due to many reasons and so I have checked speaker placement, tested various toe in combinations, adjusted the spikes etc. I have tightened the bass, a bit, but that boomy stuff is still there.
I am wondering if the sorbothane pads under the spike cups could be a factor. I put them in to stop pushing vibration into the floor, but I am not an expert in the science here and it could be that sorbothane is only an insulator at certain frequencies, i.e. not really doing much.
I am hoping that this is not the speakers, but any ideas on where to start would be welcome.

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