I saw this video where they filled the hollow front baffle shell of some ADAM T8V monitor with concrete:
The resulting playback sample demonstrated less coloration after the mod. It's not immediately obvious how the unmodded version is "bad" since it (w/out mod) apparently also increased the volume due to harmonic distortion adding a bit more "character" to the sound. However, studio monitors should be as neutral as possible so we can conclude the concrete reinforced baffle is the winner.
I also found similar deficiency in my cheaper JBL LSR monitors having this issue, but I already made a similar mod using different material.
I checked my Sceptre S8 monitors, and behind the thin front plastic fascia the MDF was already 2.75 cm thick around the concentric driver -- though not the entire height -- and with absolutely no spacing/air gap like in the cheaper Adam.
Out of curiosity, I filled the plastic trim and front fascia of the Sceptres with heavy duct sealant -- just happened to have a whole big tub around -- and did measurements afterwards. Result was very little change/improvement, if any at all. Well, I'm rather relieved they didn't skimp on the physical construction of these monitors!
I still have some of that grey sludge left in that duct sealant tub... hmmmn. might do my old Mackie (cheaper MR8 MK3) monitors next.