The V-Piano is set up to use four decent sized monitors...it outputs to four speakers (two for stereo, and two for the ambiance of the room).
I imagine Roland will recommend their own monitors...what type is anybody's guess so far.
It would probably be unwise to skimp on speaker quality, especially at this level.
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.