Don, good to hear there's room on the Harmony M's input control to effect its output level.
Gary's test will be from Harmony M unit to keyboard fixed line in (aux inputs), to determine whether there's enough input adjustment on the Harm.M to get the mic volume heard thru the keyboard's onboard speakers ranging from off to soft, as well as all the way up to 'significantly louder' than the relative volume of the auto accomp & live keyboard parts.
This would avoid needing to add an "in-line volume control/amplifer" such as the one Gary gave the link to.
Gary, I'm looking forward to your findings.