If you have a 220 VAC receiver...plugged into a 120 VAC outlet, then you are running it through a STEP UP transformer. Your receiver is then getting 220 VAC from the transformer, and outputting 220 VAC at the rear outlets. More info about your system would be helpful, especially how it's hooked up, plug configurations too. Since those crazy Europeans can't agree on one type of plug.

If this is the case, here's a quick fix; Get a voltage adator from radioshack. It's designed for people from the US to use 120 VAC appliances plugged into 220 VAC european outlets.

Unless I misunderstood your system, that is. If on the other hand, you have switched the internal power supply to run on 120 VAC, which is entirely possible, it then should be outputting 120 VAC, all you need to do is change the plug configuration.