Good point, Jim, and that does seem conceivable to me, especially if the interconnects to this amp are single-ended (I don't know if they are single-ended or balanced).
With the amps running off of 240V, both sides of the primary of the power transformer in each amp would be 120V removed from chassis and circuit ground, conceivably increasing the degree to which leakage currents would tend to affect chassis potential, at least at noise frequencies and perhaps at hum frequencies as well. And with the preamp powered by 120V, the ac safety ground wiring would be unlikely to be effective at keeping the amp and preamp chassis at the same potential, because the inductance and resistance of the run between each component and the electrical panel is in that path. So the likelihood of inter-chassis noise currents flowing through the interconnect shields would seem to be increased.
So of the several factors we've identified that could conceivably result in sonic differences between the 120V and 240V versions, the majority of them point in the direction of the 120V version sounding better. Not that that means too much, of course; the only way to judge would be via carefully controlled listening comparisons.
Best regards,
-- Al
