Excessive "hiss noise" from Audible Illusions 3A

I just brought an Audible Illusions 3A over to friend in Europe. We hooked up to a Rotel 991 amp and the combo is quite stunning. HOWEVER, there is a significant amount of hiss noise coming from the speakers when music is not being played, or at very low volumes-even when turned all the way down. The only way to make it go away is using the mute switch.
The Rotel is on 230 Volts straight from the wall, but the AI3A is a 120 Volt U.S. version-but is fed AC through a voltage converter, which is NOT grounded. Could the lack of ground have anything to do with this hiss?
I tested the AI3A with a headphone amp in the U.S. a week ago, and it was dead quiet opened all the way up (which can not be said about the Grado RA1). Which brings me to suspect the lack of ground or the voltage conversion. By the way, the hiss is coming from both speakers, and is audible from the listening position when playing music at low levels. Changing the source to phono, which uses different sets of tubes, does not change anything.
Your help would be greatly appreciated,
Thanks