Make sure it is below the digital source so that the electrons can flow easily to the DAC. ;-)
Seriously, it does not matter. Make sure it has a clean power supply, it is well ventilated, and cabling is no longer than necessary. Vibration is unlikely to cause problems with the sound of modern equipment but may affect tubes. Excessive vibration may shorten life span. Keep high voltage cabling away from signal wires and keep the unit away from high RF/EMI sources like transformers.
Seriously, it does not matter. Make sure it has a clean power supply, it is well ventilated, and cabling is no longer than necessary. Vibration is unlikely to cause problems with the sound of modern equipment but may affect tubes. Excessive vibration may shorten life span. Keep high voltage cabling away from signal wires and keep the unit away from high RF/EMI sources like transformers.