I think you are proceeding correctly with the 120V version, for the reasons stated by the others who advocated doing that.
The only technical differences I can envision that would result from running on 240V are:
1)Less current would flow in the power wiring, the ac fuse and switch, and the primary of the power transformer. I would expect any sonic differences resulting from that to be subtle and unpredictable at most, and not necessarily in the direction of being better.
2)You would be exposing the ac input of the amps to noise or distortion that may be present unequally on both 120V phases, instead of what may be present on just one phase. That difference should be minimal since the 240V wiring you would be using is a dedicated run. But to the extent that there were any differences, they would probably be in the direction of making 120V operation preferable.
Re the dealer and others claiming that 240V is preferable, I question whether those claims are based on extensive and carefully controlled comparisons that eliminated other possible variables.
Regards,
-- Al