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,
Id bet its the tubes, ive owned 4 of these and every time its noisy Swap something new in there. Those preamps eat tubes. Ive had a humm come from one channel, opened the cover and the glass was cracked out on 1. Hope you find the cause.
Arni, I doubt the 230-120v converter is causing your hiss problem. Most likely your audio systems sensitivity is suspect and the preamp needs a new set of low-noise tubes. Be advised the phono signal is routed through the preamps line stage and I recommend a matched set of 6H23EB tubes. Regards, J.Cusmano/AI Service Tech.
Thank you for your help, I will change out the tubes and hope for the best. And you were right, grounding did not change anything.