Sonical differences

Yes, but not to the same degree as an active one. I have found that even a passive preamp, which I tried a couple of years ago, still benefits from good isolation.

I suspect you could still place some isolation devices beneath the preamp whether horizontal or vertical. As always, try it and hear if it makes a difference. Cheers.

I would think that you would tend to run into different types of resonances when changing orientation. Vertical orientation would tend to become more susceptible to air-born vibrations whereas horizontal would be more susceptible to the increased coupling to the rack / support shelf.

If you're looking for the ultimate in isolation, put the components in another room. Unless the system is REALLY rocking, the amount of vibration that the components will have to deal with should be DRASTICALLY reduced. Not convenient, but probably one step higher in terms of lowering the noise floor. Sean
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Sean, again you have said it all. It may interest you to know, that I have most of my electronics out of my listening room, including turntable and CD player.

...hmmm maybe I should consider placing a few of my components in the kitchen that is bihind the wall from living(listening) room(mono-amps maybe since they need more isolation because of powerful output transformers) and I'll be only with the transport, dac, tt and phono in my listening room(I think that passive preamp doesn't count in terms of isolation)?