A common tweak for a lot of speakers to deaden and solidify the base. I owned a pair of Sonique (Australia) speakers that were factory filled with sand before my MLs. Not sure how the ML cabinet would stand up to it though - with all the electronics!.
If he paid $4800 for his Sequels I have a few things I'd like to sell him!! Having never taken the woofers out of my Sequel cabinets I wasn't sure exactly how the sand idea would work. Or if it would work.
I would hesitate to put anything inside a woofer cabinet that would change the original internal volume. A precise internal volume is a key element in the design of the speaker and any changes to it would likely screw up the sound.
It will sound "different," but "different" is not likely to be "better."