Using a 100 Volt amp on our 120 volt power line ?

Bad idea. You would definitely need a step down transformer. Running it at 120 volts could be seriously shortening component life. 

Think of it this way: would you be comfortable running a USA amp at 144 volts? If it’s possible to modify, the power supply, that would work as well as a step down device.

He has been using it here in the states for 6 months ( without a step down transformer ), claiming no problems exist, and that it sounds wonderful.

If the guys mains voltage is 120Vac he has been overfeeding the amp by 20%. Usually the max continuous over voltage is 10% of the rated nameplate of the equipment. (Check what the nameplate voltage rating on the amp actually says. Is it 100Vac only? 100Vac X 10% = 110Vac.)

Note even though the mains AC voltage may measure 120Vac at any given time of the day that does not mean there are not times when the mains AC voltage does not peak higher. The owner of the amp already has been operating the amp beyond the manufacturer’s built in 10% fudge factor.

For 6 months he has been running the amp off of ???Vac, 120Vac nominal, voltage. It is possible components in the amp have already been stressed.

As for reducing your mains voltage to 100Vac you can buy a converter. Usually they are autotransformers.

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They have 120V to 100V converter transformers on eBay. I also recommend usage of one - 120V is too high for a 100V device, especially on a power amplifier - it'll run rather hot.