How do you think vibration effects the sound? To me this is mute, the only way i know of that an electronic component will be affected by vibrations is if there is a magnetic field in the area strong enough to induce current in the system due to said vibrations. There are micro magnetic fields everywhere and some even higher ones due to sun activity, etc. but even those can be easily filtered out in electronic systems, ie feedback, shielding etc. If you're not getting hum in your system from hi magnetic fields (note usually grounding issues), I don't see how the amp inside or outside makes much difference if designed well.
Thats my take but i may be missing something or maybe.