Sorry, Hammer, but if you heard a noticeable difference in stereo between the mixer and going in 'straight' from the keyboard, you have messed up the wiring when going 'straight. Sure, the mixer will change levels for you, and EQ's, but it can't do a THING for the stereo spread. Stereo is stereo, plain and simple. L goes to L input, R goes to R input. That's all there is. Onlt thing the mixer COULD do is narrow the stereo field (by using two channels and panning them closer to 12 o'clock).

But you can't GET more 'stereo' than taking the L output of your arranger to the L stack, and the R output to the R stack.

Doublecheck your wiring. Something's not right...