DAC placement

I have found that isolating them from vibration etc is helpful, a seperate shelf would be good but not essential. I have my transport and DAC sitting on top of a roller bearing platform which is itself sitting on my Star Sound rack. They are seperated by a set of Ceraball isolation devices with a VPI Magic Brick sitting on top of them. Probably overkill but sounds good. I have found that most componets sound better with a weight on them, does not have to be exotic or expensive.

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.

I think that any sound isolation or coupling is going to be more important on the DAC than the transport because any effects will be stronger in the analog signal. I know that at my local Hi Fi store they had Nordost Sort Kones under the PS Audio DAC but not the matching transport. I'm convinced of the difference that Sort Kones made for my SACD player, but I would attribute it to the D/A portion of the player.