No, going through a mixer doesn't work when doing this type of comparison because you still have those plugs plugged into the back of the keyboard. Within the Left/Mono plug and the Right plug, there are leaf switches that direct the signals. Consequently, this prevents you from playing in mono if you merely turn off the amp on one side of the mixer or turn down that channel in the mixer going to one of the keyboard's outputs. When you plug a jack into the Left/Mono and nothing into the right output, the leaf switch connects the outputs together internally to that single output. However, if you plug into just the Right output and nothing into the left, then you would only get the sounds from the right channel and nothing from the left.

Jim, you can always call me on the telephone if you need help with any of this stuff. This time of year, I'm around the house most of the time. I will PM you the telephone number.

Gary