You probably have a power imbalance and you are using too small of a transformer for the task. This causes transformer core saturation on either the positive or the negative AC cycles... leading to harmonic distortion and a DC offset that induces noisy ground loops which are easily picked up on RCA interconnects.
Your amp uses something like 240 Watts when run at 220 volts suggesting it will need around 500 watts at 110... so I would use at least a 1000 watt transformer to be on the safe side.
Your amp uses something like 240 Watts when run at 220 volts suggesting it will need around 500 watts at 110... so I would use at least a 1000 watt transformer to be on the safe side.