Best little and inexpensive (well under $200) speaker system I've used is the Logitech Z-2300,,,perfect for keyboards...just the right amount of bass without being blatty or tubby, and quite loud as well. I use one on my PC and also on my PSR-S910 and T3.
The Roland KC-110 might work okay for busking, but the one Fran got, the 330, would be the better choice.
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.