This has been discussed here a lot already. Search through the forum for Bose or Stereo discussions and you'll find the info. Short summary: stereo PA or amps sound better for keyboards if you want to go to the expense (yes, in the case of the Bose system you would need to buy two for stereo). If you go mono, there's a possibility of phase cancellation affecting your sound. Some people can live with that, some can't.