I remember reading somewhere that the Tyros I voices were sampled at 48 kHz and 16 bit (could be the owner's manual, could be an Internet site, I can't tell right now). I think that the Tyros II voices have the same sample rate, which is more than adequate for live or studio use.
The difference you hear in sound quality between Tyros I and II could be due to the different Megavoices used or could be simply an artefact of the recording technique used (a bit more of reverb or chorus, etc); sometimes they record something in mono and then another thing in stereo, to impress the listener. I wouldn't form an opinion on the sound quality until you are able to play the thing in person.
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