Tube amp on the floor or on the rack?

You might want get some type of cage for that thing, if it doesn't have one. Kiddies and pets don't do well breaking tubes and burning themselves. (I don't know your situation). The only problem with "up high" is-it can fall that much farther, not to mention when the heaviest piece is on top it will create more instability should the rack get knocked. Keep it off the floor so it will need be ventilated. And a good amp stand will do that and provide some vibration immunity, of course as heavy as it is its own weight will help that. And some type of cage protection would help.