That old PSR-5700 had the best piano and vibes sounds Yamaha ever made. They were stellar to say the least, but discontinued when the lightweight systems came into being. The only person I know that still has one is Eddie Shoemaker (btweengigs). It was a beast, had a builtin 50-watt RMS amp, 6-inch speakers, and tipped the scales at somewhere around 55 pounds. When I had one stored in a semi-hard case it was like lifting a refrigerator. But damned it sounded good. Unfortunately, it was very limited in what it could do.
All the best,

Gary