step down transformer recommendation 100v to 120v

I own a pair of Exclusive monoblocks M5a that were design for 100v (japanese market). My preamp and cdplayer is made by Accuphase and they also run in 100v mode. I am wondering if there's a step down transformer you can recommend for this kind of equipment. I also hear that units made for 100v can easily be 'redialed' for 120v. if so, how can that be done? I am aware of a few websites in the U.S. that offer step down transformers but I am not familiar with their reputation. Can a transformer affect the sonic quality of this kind of equipment? is there such thing as as a hi-end transformer of this kind? any help is welcome. I have checked out a few threads on the web about stepdown transformers but they seem pretty ambiguous if any. when the question about "sound quality" degradation comes up it seems like the answer rather dodges the question. anyone out there with solid experience in this arena?
the current transformer I am using seems to work fine. however, at times, it seems to make this sort of "white noise" whenever i run the amp at high levels. perhaps i am asking too much power at once from the transformer. my amps are rated at 600w per channel drawing 400w each from the outlet. I forget the preamp specs but i think it is around 200w and the cdplayer is close to that too. any help on how to buy the right kind of transformer is welcome. thanks!