Running a synth through an amp adds a certain quality or color to your sound that you control(reverb,didtortion,EQ).
If your amp has a "Line Out" on the back,your soundman can take a DI from there,leaving you with total control over your sound as before...
If no "Line out",then your amp can/should be mic'd similar to the way a guitarist's amp is done.
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.