I have decided to forego purchasing the Japanese unit.
Sorry to hear you have been frightened away from your plan. I agree with Pauly that bigger is better. Most Transformer cores saturate when used close to their rated power and that is when you get annoying humming from DC offsets and potential degradation in the quality of the power.
I am actually quite surprised that the Japanese unit will not function with 120 Volts if it is rated at 100. I would have expected the Japanese device would tolerate the slightly higher voltage. (Nominal power is allowed to vary about 5% from your utility and devices are normally designed to function properly a further 3% to 5% outside this range (due to further voltage drop in the home). I would expect a similar kind of tolerance for Japanese equipment.
Signal degradation is very unlikely.