[GaryProtein]

Caution! Is 20 inch depth enough for the power amp and other components WITH the XLR cables and connectors still being attached?

This is VERY important since you are having a backboard made.

A friend of mine had almost $10,000 worth of custom cabinetry made that didn't fit his components because he gave only the component dimensions and neglected to figure in the additional inches required for the XLR connectors, which with fancy cables could easily add six inches in depth for the cable bends behind the units.

Without knowing what components you have, I would think about making the shelves several inches (probably six inches) deeper or not using a back board.

Your 38 inch internal width may also not be enough if your components have 19 inch wide face plates. You should add at least two inches to the internal width so you have space for two 19 inch face plates PLUS 2/3 inch on each side and between the two components.

One inch thick granite slabs weigh about 15 pounds per square foot (171 pounds per cubic foot), so make sure your substructure can handle the weight.