I have the PX-560 and 2 Roland Cm-30 speakers.
The build in speakers are 8w + 8w (stereo). A single CM-30 is 30w.
The speaker in the CM-30 is larger than the ones in the PX-560 so in my opinion it overall sounds better than the internal speakers (e.g., covers more range and volume.)
I do think the internal speakers would be enough for very small gigs where the room might not be too loud.