What are you trying to accomplish with your sound? What is lacking in the external amplification compared to the PA-800 internal speakers? Are you aware that your internal speakers do not reproduce frequencies accurately, but are a rather cheap convenience.

You have to adjust your sounds, styles, performances while listening through your actual PA system. In most cases, once it sounds good through the external PA you will notice that the sound is very crappy when reproduced on the internal speakers and vice versa. However, keep in mind that different venues will sound differently - room acoustics. You don't have this problem when you use internal speakers, because the sound source is right in front of you and mostly unaffected by the environment.

Instead of wasting money on different speakers, I would advise you to spend some time adjusting your keyboard settings. Remember to take breaks after every hour of listening and tweaking, as your ears will likely start to favor certain frequencies over others!