DIY concrete stands

I've been fooling around with DIY stands made of concrete block.

I first made two speaker stands for my gf. Each stand was 3 blocks, sealed together with white grout. I used a thin wood veneer on the bottoms, to which I attached felt sliding feet. On the top, so far, I just have a thin cardboard piece, and the speaker resting on that.

I planned on wrapping a fabric around the stands to hide the concrete block work. The funny thing is, they look really cool. They're on wood floors, with rich tapestry burgundy/gold curtains behind them. And with almost no gap between the blocks, and black Paradigm Studio 20's resting on top, they look more stark-modernist than white-trash. Weird. My gf won't let me cover them with fabric.

I also made myself an amp stand out of two solid concrete blocks. I have the blocks sitting on a thin cardboad pice (so they don't scratch the floor) and my amp sitting on styrofoam peanuts (crushed beyond visibility) on the blocks. I don't know for sure if there is a difference in the sound (it's a ss integrated, after all) but it's kinda cool. Again, it's sitting in a refined and polished surround. In that setting, with a pretty amp on top, the raw concrete slabs actually make a blunt aesthetic statement.

Anyway, the only thing that's cheesy about the speaker stands is the feet (the felt sliders) and the top (cardboard, right now). They look cheap (and they were!). I'm wondering what I should do. I'm thinking of getting thin steel plates, which I paint black and attach to each end, then fixing adjustable feet to the bottom, and mounting the speaker with blu tack on the top. But where do I get adjustable feet? Especially cones or spikes?

Even with the felt sliders, these stands are solid! And total cost was $25 bucks and about 2 hrs of labor (me... clueless at that).

Is there anything wrong with using conrete to stabilize gear? Mass is resistance to acceleration, and it's a lot of mass... cheap.