In any standard hi-fi system you are not powering the speakers, you are powering the amplifier with the mains power. The amplifier(s) may of course be built into the speakers. Speakers are normally rated in terms of Watts RMS (Root Mean Square) and the rated figure is the maximum they are designed to take at any moment.
500W RMS is approximately equal to 300W steady state so a step-down transformer rated at 500W would be more than enough for the job. Further, amplifiers run on DC which is rectified and smoothed from their AC supply - in the case of audio amplifiers, very heavily smoothed ! So the difference in phase between 220V 50Hz and 110V at 60HZ is not important for your amplifiers. Where it might make a slight difference is in the way the internal transforming circuit transformers heat up, but unless you plan on running these speakers at Max. all the time there should be no problem.
To sum up, if you want to take these speakers with you AND they are not fitted with an AC voltage change-over switch, then you will be perfectly fine with a dropper transformer rated at 500W/220V - make sure it is properly earthed.
One final point. If you are coming to Europe they mostly use a round two or three pin plug very similar to the US model but I believe the dimensions are slightly different, it would be worth checking. Of course, if you are coming to the UK - we use a completely different design of three pin plug, which I believe is used nowhere else in the World ! At least we now use only one design of our three pin plug, we used to have several different sizes - none of which were inter-changeable !