I am glad to hear that you are enjoying the upgrade.
See if you can find some type of heavy industrial floor tiles. Line the cabinets with it then add about a 1" open cell foam layer. You can then tweak the tuning with a light amount of fiberglass insulation or polyfill.
I have some floor tiles lying around. I presume the idea is to add weight to the enclosure to reduce resonance. But how do I affix it to the MDF (normally in India we use white cement to fix it to the floor). Won't this reduce the inner volume of the enclosure.
Can the same me done on the outside. i.e I mean can I fix the tiles instead on the exterior and add the fiberglass to the inner walls.
One more clarification, there is one more cap and resistor in the crossover, can I swap that too with a higher quality one?
In one of your posts in this thread where you have completely rebuilt the crossover, I see you have used rubber speaker terminals. What exactly is that?