Quad Tone Rip is an alternative printer driver than allows you to control the specific ink limits of each channel, how each channel overlaps, and the linearization of the overall grey curve. It allow you to break away from and not rely on manufacturers settings which might not provide print quality that is acceptable to more advanced printers.
It also allows you to blend up to three ink "recipes" to finely control the final print color, even allowing you to split tone in the shadows/midtone/highlights.
You need a photospectometer to easily create custom profiles, but a densitometer will work as well but will just be more labor intensive. A scanner can be used for some of the curve creation process but wont be as precise and might cause more trouble that it is worth.
I am writing a book about it, and will let you know when it is available for purchase.