I agree with the others. Adjust the master EQ on your 500. You'd be surprised what master EQ adjustments can do for any keyboard. If you're just using the speakers at default you'll notice a HUGE difference when you adjust the master EQ to your liking, and I would be very surprised if the 500 didn't have preset master eq settings as well.
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GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.