Just to add a bit more info, I neglected to make it clear that both the enlarger light source and the Macbeth light source are fluorescents. I found a thread on another forum that mentioned how the 2-color Gossen Sixticolor does not handle fluorescents well, OR dim light situations ("dim" was not quantified).
I think you're right on & when you get your answer from Gossen it will be verified.
I don't think the Sixticolor was ever intended for use with flourescents.
Flourescents aren't full spectrum lights any how
After querying both Gossen's US distributor and Gossen itself (relaying most of the text from my original post) requesting any information they might lend regarding my meter's responses, as well as inquiring about whether the Sixticolor might need adjustment, I've received no response from the distributor and today I received the following response from Gossen:
"The production of the Sixticolor meter had been stopped many years ago and
we kindly ask for your understanding that we cannot service it any more,
neither here in Nuernberg nor in the USA."
While this is not surprising, it is obvious that this forum has provided FAR more food for thought. And I thank you all for that.