I’ve experimented with weights on top of speakers in the past, not on springs and found that it’s easy to overdamp the cabinet. Not always a good thing.This why God gives me ears: to adjust the weight of the damping mass.... :)
And i use the same ears to adjust the compression of the springs by adding mass to the speakers... It takes me 3 tryings... First not enough: then harshness accent on high frequencies, after that too much weight and darker sound too much accent on the bass or mids bass....
The last one experiment were perfect playing with 2 mass of 5 pounds ....The critical difference were around adding or substracting 5 pounds of weight on 100 pounds of total mass(speaker mass +concrete slabs)...