Although large-format printers will operate 4-8 ink cartridges (starting with CMYK, at least), their computer brain RIP is designed to receive and process RGB color information. It decides how to translate it to its physical cartridges of ink.
So, if you feed it CMYK numbers, it likely will convert it to RGB numbers, and then RIP it to operate the CMYK cartridges. That is introducing a double-conversion that is effectively limiting the fullness of the color that could be.
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