Why buy separate pre and power amp and then stack things. A simpler solution would be an integrated amp.
There is a whole industry out there working towards reducing noise (electrical, mechanical, etc.) - from racks to footers to damping material.
It is like walking bare foot vs wearing shoes.
I have found Isol Pads to work positively and universally under all audio equipment - source, pre-amp, poweramp, cables, speakers. And they are quite affordable too (4 for $25).
Cheers
Sunil.