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.

Look up the given model on hifiengine. The tech manual will generally have the transformer schematics for the different regions.

Some models from some companies come with universal transformers, where the taps on the transformer only need to be swapped, and some come with transformers that cannot be re-wired.

With those (not re-wirable) you should use the appropriate step transformer.

Besides, you get notably better sonic qualities with a transformer in the system on the AC side.

Being forced to use a Step up or step down transformer is like being forced to have higher quality sound.

’tis a ’orrible thing. No one would want it. Better quality sound. Bah!

Actually there is a good chance that the amp is already degraded by 
running it at 120 volts. 

Hi all. After more than a year of posting this thread, I am planning on purchasing a 100 volt cd player from Japan, and I am looking for a suggestion for an excellent quality, proper converter, for use here, with our USA 120 volts. Under 100 watts would be safe. Thank you very much. Enjoy ! MrD.

A simple way to deal with this is something called a 'bucking' transformer'.
The idea is pretty simple, and its cheap. You need a transformer that can do about 15-20 volts at its output under load. It must be able to prvide the current needed by whatever is running off of it (in this case, a CD player, which might draw about an amp). So that means a fairly small and inexpensive transformer.

I can explain how the transformer is hooked up, but it should be done by a qualified technician. This is a lot cheaper than getting a stepdown isolation transformer and it works just as well.

How about a variac? It could be adjusted to his actual line voltage which can vary from one area to the next. They're available with integrated volt meters at a very reasonable price. Just a thought.