Experiences with mass loading stands

Yes you can overdamp anything in audio. This may be your problem. Your description fits the general impression of an overdamped element deadening an otherwise lively sound.

What is the material between the bottom of the speakers and the top plate?

I might have somewhat expected an overdamped sound had I filled the new stands all the way up, but they are completely empty right now. Hence my question above as to whether damping the stand will bring positive benefits vs. overdamping the sound even more.

Speakers are coupled to the stands using Herbies dots. Haven't tried anything else yet.

As you have found out, the stand can have a profound effect on the sound. Remember that you dialed in the speakers on the old stand for best results and you probably placed the new ones in the exact same place. You have to start over with placement. Also, the MRs are not difficult to fill if you use dry cat litter instead of sand, which is a PITA.

Also, you need to experiment with the interface between the stand and the speaker. If you have kids, you probably want to use Blu Tak, which prevents the speakers from being knocked off the stands, but if that's not a problem, you should also try other things like Herbies Black Dots or small wood blocks, until you find something that works.

While every system is unique, and it's difficult to know how a speaker will click with a room, I've never heard a flimsy stand sound better than a good one. If the flimsy one gives better bass response, you may have a real boom problem in the room and the cheap stand is countering it by inhibiting the full bass response of the speaker. Now you put it up on the better stand and the bass is more pronounced, which is muddying up the rest of the spectrum. Good luck sorting it all out and try to have fun experimenting.

Target stands aren't what they used to be, but if you want to compare these two stands you will have to fill the Target stands.

Different speakers need different stands to sound their best. ProAc speakers need a very heavy stand. Epos speakers sound their best with a light weight stand.

Trial and error is the only way to know for sure. Then you can report your findings to us, so we all will know.