@mahgister I can see from your description about mounting a Speaker Cabinet you take your investigations into tidying up a sonic quite seriously.
I have a substantial supply of Cork Pads, they regularly find a place as a additional tier in an assembly. My interest in Granite has a 30ish year history of using it in multiple configurations in a support structure.
On my Cabinet Speakers my the Top Weight is a Large Lead Metal Block rested onto four 1/6" Cork Pads. The Lead Blocks then has approx' 20lb of Steel Dumb Bell Weights sat upon it. Moving this weight around to different resting positions on the Top of the Speaker does have an impact where sonic or box coloration are able to be impacted for the better/worse.
I am a ESL Speaker user as well, so the Cabinet / Driver issues are easily by passed if wanted.