I compared the Genos2 + GNS-MS01, SX900 and SX700 at a dealer a few months ago. The Genos2 with the GNS-MS01 sounds better than the SX keyboards with the built-in speakers. But this is to be expected and is primarily due to the sub-woofer of the GNS-MS01. How much better is of course very subjective.
Don't get me wrong, the SX700 and SX900 sound very good. If you connect a sub-woofer to the SX900 or SX700, the difference in sound compared to the GNS-MS01 is, in my opinion, more a matter of personal taste as far as the speakers are concerned.
And of course your age plays a role. In general, by the age of 50, the ability to hear high-pitched sounds has already decreased significantly. The position of the speakers can play a more important role then.
Kurt