Well, I seriously hope everyone at Yamaha was using an $50,000 "mastering-grade" system while working on the XS, lest they not have a good enough reference to evaluate their engineering/sampling/voicing...! ;-) FWIW, I usually use high-end headphones for evaluating synths--of course this does can expose too much detail when listening to individual patches--e.g., aliasing that would not be of much consequence in part sitting back in a mix.

I think you're really talking about aesthetics here--a lot of synths have a certain sound to them that is "imposed" on all sounds--presumably down to the synth engine/"tone generator" itself (Kurzweil K2XXX synths IME always have a "certain" sound), the choice of samples and the voicing (e.g., I like Korg workstations, very "impressive" sounding, but perhaps a bit hard and "brittle" in excess doses; but in any case, the overall "Korg" sound seems to often apply to the acoustic instruments such as pianos etc.)

If there isn't a necessarily consistent aesthetic--for instance, I think the Yamaha "MegaVoice" guitars (in the ES/Tyros/PSR-3000) have a more "natural" aesthetic to them that may not be the case with all sounds in the ES--then of course if you're after synthetic sounds then acoustic guitar demos may not contain much useful information--but then again, it might suggest that you won't like those acoustic sounds on that workstation, or they are not suited to certain styles.

I don't think this "character" is just a matter of sound "balance," and it might be quite audible on a medicore or even downright bad system. (The other day I caught a bit of an (idiotic) "reality" TV show where some kids were playing drums to a "Jump" backing track, and it was obvious that the synth brass was NOT coming out of an Oberheim--or presumably any other decent synth for that type of sound--there was very little depth or "movement" to the sound--some would say it was "thin"--even though I happened to be listening through very poor quality internal TV speakers.)

IMO often the "character" of a synth does come across well in demos, sometimes flaws do as well. What demos inevitably can't show is the full potential of the synth for your own applications, be it in terms of the specific patches/presets demonstrated, or sounds that you could program yourself, etc.

(Consider, though, that not everyone online lives in a country/area where it's particulary convenient--if possible--to use a product in person before ordering.)