A quality step down transformer will make no audible difference to the sound. But as someone mentioned if the amps are Accuphase it is not difficult to convert to US voltage. It's the same with some other non US manufacturers.
What to expect if I import an amp and preamp from a Japan seller via US eBay
I can’t afford the cost of the amp and preamp I’d like to buy. I’ve discovered I can pay just less than half what I’d pay at a dealer in the US. But I haven’ t done this before. I’m aware that I won’t have a guarantee and that I’ll need a step down transformer. I willing to take the risk of no guarantee. But I don’t know what to expect when my box hits CUSTOMS. What can I expect? What are my responsibilities? How much is it likely to cost me?, I’ll be paying $7000 for the amp and the preamp..