I have no experience with using external speakers, so I can't recommend anything, but there a few things to be aware of:
(1) The keyboard has a Master EQ function with different possible settings. You might want to look into those and try out the settings to see how they affect the sound. You might also want to look into an external equalizer, so you can adjust the amplification of different frequency ranges beyond what the Master EQ function of your keyboard allows.
(2) Not all amplifiers and speakers are alike as far as which frequency ranges they work best with. From what I've read, you'll want to avoid guitar amps, because they're designed for higher-range frequencies and the lower-range frequencies will suffer. Bass amps are supposed to be much better for keyboards, as they're designed for lower-range frequencies but even so the higher-range frequencies won't suffer. You can also buy keyboard speakers, but I don't know how they compare with bass amps. And there are studio monitors which give a good response across a wide range of frequencies.