Not an easy problem. Most transformers you see available commercially are pretty poor not to menation most hum audibly enough to be heard from the listening position. I did use a step up transformer successfully to test an amp (I have 240v at my stereo stand but wanted to test something else). it worked and sounded almost as good. a bit of advice, overisze your transformer, use at least 2kw, preferably 5kw. A commercial transformer has a high output impedence which limits the amp. Better would be to get a power regenerator from PS audio and put it downsstream of the transformer....it has almost zero output impedence. I asked them if they made one that went 120 to 240 or vice versa and the kid on the phone gave me a bs answer why it couldn't be done.
good luck,
Jerry