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

There is a gentleman selling a high end Onkyo integrated amp on another site, which is a Japanese model, to be operated at 100 volts. 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. He does say in the ad that although he has had success with a few Japanese, 100 volt units, powered by our 120 voltage, he recommends the buyer to use a step down transformer ( he explained, to protect himself if a problem arose ). I guess my question is : Can this work without causing a problem to the amplifier, or cause a fire ? Enjoy ! MrD.
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.