Carlos
Trying to create a QTR curve with a particular toning for a K3 inkset, including neutral, by changing how much of which colour inks are used is a challenge that only a few people seem to have the skills for, and I'm not one of them. I had a problem with my OEM 3880 in that the QTR neutral curves are not really neutral. They're ever so slightly warm, not objectionably so, but they're not neutral. I "cured" this by linearising the neutral and cool curves and then blending them in QTR roughly 85% neutral and 15% cool. The proportions will depend on the paper.
This is a trial and error process that I simplified a little by using a layered TIFF. You duplicate the image and convert one copy to the warm ICC and one to cool and then back to AdobeRGB and then stack the files in a TIFF and play with the layer transparencies. This assumes that you've created and ICC for the curves as part of linearisation.
That all said, I'm a little surprised that the P600 curves that ship with QTR are not neutral. The P800 curves have a reputation for being so. I wonder whether the P800 curves would work in the P600 give the similarities of the printers.