Hello mvoelte,
Leave the switch at 8 ohms. Depending on how Sony implements its protection circuitry, putting the switch to 4 ohms could impose substantial limits on output power to all the channels, which some manufacturers do to keep excessive current flow at 4 ohms from overheating the output stage and shutting down the receiver. Some receivers rated at 80 or 100 watts into 8-ohm loads produce as little as 30 watts into 4 ohms because of current-limiting circuitry.
Normally, any receiver that drives 8-ohm loads won't have problems with one 6-ohm load.
Regards,
