why don't you get a 120 to 100v step down transformer? just get one large enough for the power draw, usually shown on the back close to the power plug. I'd aim for twice the rating but generally a 1000w step down transformer should be sufficient.
I've run Japanese 100v gear (Luxman in my case) on one for years no issue. just get a quality one.
Then your not trying to change transformers in your amp, unless it has the windings already then its a dealer/tech job.