How to properly put audio equipment on top of an antique buffet cabinet?

I'd consider either marble/whatever, or even glass, which would show off the woodwork. As noted, felt or similar is probably the best to protect the wood.

G

1 piece of granite or marble or synthetic.
coner the bottom with felt furniture pads from home depot. All good. 

Chances are pretty good that the cabinet has a laquer finish on it as most older pieces have. No matter the type of platform you elect to go with as all suggestions will work fine you will need as large a felt 'foot' as you can fine, 1"-2" if you can find them. Do not use rubber feet as they will leave marks pretty quickly in the finish. Might I also suggest moving them slightly each month the size of the felt that you are going to use. Idealy you would want to continually move the 'point load' of the equipment and platform as even these felt feet can be pressed into the laquer finish and leave a mark over time.

If you use the cork/rubber feet just cut a square washer out of felt or thin cardboard, to place between the rubber & wood top. If you use the larger feet you would not need a platform, just put them under the component feet as the larger size will distribute the weight over the top of the furniture.

Cork is not a good idea since it retains moisture. If you use felt, make sure it's synthetic with no wool or fur in it.

I generally won't put a heavy unit on an antique regardless. Best to have something that you won't worry about scratching or marking.