I'm thinking about buying some external monitor speakers as well (for home use only), but I just can't decide. The thing is, I have no opportunity to check them out before buying. Yes, I saw "reviews", but it's like buying shoes: nobody can tell how they will feel on your feet.
I think SX700/900 has good enough speakers built in -for practicing and for small family party at home. For anything more than that, external speakers are a must have. One reasons why's that, is power. Everyone who knows a little about constructing high quality speakers knows, that saying "power is 10W", means absolutely nothing! To get the impression of speaker quality, more parameters are needed: sensitivity, sound pressure level, F3 frequency response, etc. In short: if comparing two speakers at power of 10W, it can easily happen that one will be twice as loud than another, and at much better audio quality at that. Power is "cheap" -no wonder everyone brags with it.
In that sense, I wouldn't say keyboards speakers (amplifier+case+speakers itself) have some "great" audio reproduction. But as said, it's good enough for enjoying at home.
However, for those who are into music production or are playing music in public, external monitor speakers (at home), is a must have. Because only in this case we can get the impression of how music will sound on stage... stereo reproduction for example: how are keyboard instruments (voices) placed on stage. And then there's frequency response.. Built in speakers can hardly reproduce low frequencies at even decent quality and so it can easily happen we crank up the volume of bass guitar in the style. But when such style is then played on higher quality speakers, bass (or drums) will literary blow away clothes of the audience.
I think, for reasons I've mentioned (and there's more), that's why TOTL keyboards have no built in speakers -because it's expected, that the owner will use (well placed) external speakers anyway.
Just sharing my thoughts,
Bogdan