So......what do you use under your speakers?

Large Tupperware boxes filled to the brim with beach sand. I was warned by a speaker maker not to use this with his speakers. So, of course, I tried it out -- excellent.

Like you, Nobrainer, I have a concrete floor covered with commercial carpet and no padding. A couple of years ago, I bought speakers with plinth bases (no spikes). Since my floor is not flat, the speakers wobbled. I tried using shims and pads to level and stabilize them, but I was not totally susccessful. In the end, I reached out to Sound Anchors. For $325 shipped, in about four weeks, they made me custom sized cradle bases with three-point adjustable spikes. The bases are solid metal and heavy, and fit my speakers perfectly. I noticed, after installing them, the very sort of improvements you are looking for: Cleaner transients, more stable images, and a slight increase in clarity. A great value.

I replaced the stock screw-in feet on my speakers with the Grand Prix Audio Apex footers. They're a blend of carbon fiber and some sort of polymer/sorbothane, as I recall. Pricey but very effective, and allow for leveling.

I replaced my speaker spikes with Adona extra long speaker cones with adjustable tips. I also bought the cups to put the cone tips in and put a slab of 1 1/2 inch thick black granite under each speaker. It got rid of all traces of boominess and really tightened things up.

Nobrainer, it seems to me that you really need to try solutions to your problem rather than following other's advice. If possible, I would include the StillPoints Ultra Fives perhaps on their old Component Stands and the Star Sound Apprentices. These have philosophies that are quite opposite.