Who makes custom sized BB & granite slabs?

You might be able to find something at Grizzly Industrial.

I had the same spkrs for almost 4 yrs. & wound up powering them with Ayre V-1xe. They sound as good as what you run thru them.

I also used 7 Herbie's Footers under ea. module: 4 under the heavy side, 1 in the middle & 2 on the other side. These are only 1/2" high & 1" sq & the modules look just right. The best part is it effectively isolates the bass from the top, which gives a cleaner presentation.

I have a real wood floor, meaning there is a true sub floor below the wood floor and I have two layers of pad under carpet. I had one-piece SS spikes & had excellent bass. I think in your cement floor situation, isolating the spkrs completely would be excellent & allow even tighter bass. I would try the Herbie's stuff (or similar) in addition to Aurious. I've used both under components & liked both, so it's just a matter of what type of isolation works best in your set-up.

Looks good!