Can you do this?

can you stack a stellar dac on top or under a S300 power amp or is it best to keep them separate?

While it’s probably always better to separate everything, I have the SGCD under one of my M700’s and it seems to work quite nicely, likely be the same for the S300

Should be fine. Remove the feet and install Isoacoustics Orea’s I have them under everything but my BHK 300’s.

Not a problem at all. You won’t have to worry about heat or noise interference. Like Watchdog suggests, some thicker isolation feet is always a good idea!