We've gotten this question a bit over the last 31 years of production of our two piece preamp. Its very clear that if only one anti-vibration platform is available, it should go under the preamp section as opposed to the power supply chassis. One thing to keep in mind is anti vibration platforms have an upper weight limit as to how effective they can be; depending somewhat on the means that it employs to do its damping. So if your power supply is particularly heavy the platform may be less effective damping it as opposed to the lighter preamp section.
In our case we found this to be audible and easily measured. Using a sine generator, find the resonant point in your room (if it has one; many rooms do) and run that sine wave through the system and do the measurements with a mic at the listening chair. Compare that with and without the platform and you will see the results easily enough. We found less energy in the room at its resonant point and less distortion also ((our preamp is tube-based; less microphonic feedback). In listening we found the bass to be more solid and overall a more transparent presentation.
Thanks.
