Alain,

If you have a Roland KC300, you already have a 4 channel mixer onboard. Since you only have 1 KC300, your output will be in mono. If you want stereo, you need to get another KC300 and connect them with the link cable. But one KC300 works just fine.

Connect your mono output (usually L) from your KN6500 to Channel 2 (Left/mono) on the KC300, and then connect your VA5 to Channel 3 (Left/mono). Adjust the volume knobs 2, 3 and master. This should do it for you...

Henry