Running a subwoofer with a transformer at 50hz will be fine. The only issues that really are concerns with 50/60 hz is when there are electrical motors that rely on the hz to control speed or compoents that need the hz to determine timing, such as motors on a turntable.
120 volt stuff such as amps and preamps will work fine with a beffy enough transformer. Remember to multiply the amps by the voltage to determine the total watts needed in a transformer. For example if your subwoofer is has a 1000 watt amplifier built in, the power ratings might be something like; 9.0 amps, 120 volts = 9x120=1080 watts. You will need a transformer rated at least 1300 watts, however to be safe, I would run one with at least 1500 or 1700 watts.
