A slab on concrete will be solid. I was thinking of the suspended wood floors many of us have up here in the north.
My gear is on a Sound Anchor component rack spiked to the concrete floor in the lower level of our house. Consider the Sound Anchor rack. Solid as it gets and holds value. Since you will be in a closet, the castors could come in really handy when trying to change out cables or components. Also, they do not have shelves but instead the gear sits adjustable steel bars providing both flexibility of footer placement as well as better air circulation beneath the components.
My gear is on a Sound Anchor component rack spiked to the concrete floor in the lower level of our house. Consider the Sound Anchor rack. Solid as it gets and holds value. Since you will be in a closet, the castors could come in really handy when trying to change out cables or components. Also, they do not have shelves but instead the gear sits adjustable steel bars providing both flexibility of footer placement as well as better air circulation beneath the components.