As a long time stereo builder (1975) and PC builder (1995) I would highly recommend against ever putting equipment on the floor. That's where all the dust, dirt, bugs, and water will congregate. Also limited air flow. I was taught 16" minimum , that's the height of electrical receptacles in most houses.
I will be the first to admit that it is sometimes not practical or feasible to do. I just bought a new VTI HIFI rack. I was replacing a cheap POS Wayfair stand. I spent right around $1000 for 10 racks. So even thought the bottom 3 racks are only 4.5" off the floor I am using them for my power amps. VTI makes power amp stands for mono blocks.
I can't say the sound has improved but my peace of mind has. I kept waiting to hear a huge crash and finding all my stuff in a pile. New rack is rock steady and looks really nice too. Hopefully last one I will need to buy.
I too started with cinderblocks and 2 x 8 floor joists for shelves. Then I moved on to schedule 40 PVC filled with sand. 1/2 " for speaker stands and 1" for actual rack. Sturdy with no vibration. Then I got married and wife insisted upon "real" furniture.