My guess is probably Yamaha PSR-9000 to the original Tyros. Mega Voice made it's first real appearance on the latter, and has defined Yamaha's great style realism ever since.

Still, the 9000 was significant in having 4 Variations per style, as opposed to the two on the PSR-8000, so it's a tough call.

Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.