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.