Under my tower speakers -- Isoacoustics Gaia, other options?

Since vibration control is not really an issue with concrete floors, you may want to try a speaker platform like the Symposium Segues I use under my tower speakers to clean them up from internal vibrations inside the cabinet. They can be custom sized to extend beyond the speakers by a few inches on each side to give a little more stability if they came with some kind of outriggers.

Although mine sit on a wood floor which on top of a suspended floor over a crawl space, the owner of Symposium Acoustics said locating them a few inches might be a negative and the platforms provide some isolation as well. 

Peter is a great guy, very helpful and tells you when his soluti9ns won’t help, and offers a money back guarantee (unless you get them custom sized).

In my mind at least, I do believe that the isolation platforms not only help eliminate the floor form adding to the sound but also vibration getting back to the speaker, especially with concrete floors. The vibrations have no where to go but back into the speaker. That’s how I think the iso platforms would benefit your situation. 
Kind regards. 

@nonoise 

On the contrary.

If the T Podiums have such profound effect on something as delicate as a stick with a motor attached, imagine what they can do with vibrating box!

@whiznant 

 isolation platforms not only help eliminate the floor form adding to the sound but also vibration getting back to the speaker, especially with concrete floors. The vibrations have no where to go but back into the speaker.

That comment, especially the bold part, has really helped me see why the "concrete floor" factor is not the end of the story. Thank you! I suppose I figured that the floor was so inert, it wouldn't resonate in sympathy with the speakers -- and indeed it would not. But the vibrations coming from the speaker is the key, and thank you for pointing that out in a way that made sense to me.

@sokogear Thank you for your recommendation. It's hard to believe a platform that inexpensive is out there as a solution. I appreciate it.

@lemonhaze I deeply respect your opinion and experience. I will read the link you provided and seriously consider the platforms. It defies my economic common sense to spend that much but you're making a salient pitch, here.