Well, it was something simple after all. In fact, it's embarrassing, and I could delete my question. But perhaps it will add to our database.
I had chosen the "Warm" curve for Curve 1 of the C6 VT inkset and I chose "None" for Curve 2. But from a previous job, under Curve Blending, I still had entered 50% for the warm Curve 1 and 50% for the neutral Curve 2 in Highlights, Midtones and Shadows. Even though I had unchecked the box for Split Tone Curve Blending those false entries were apparently enough to weaken my print.
So, I deleted all references to Curve 2 and my prints came out fine.