Hi John,
If I'm not mistaken, the P800 and the 7890 use the same 8-channel inkset. If that’s the case, it might be interesting to start with that generic neutral curve that you liked on the P800 … and then just tweak it for the 7890.
On the topic of neutral prints, I've spoken with a couple of very well-known photographers who specialize in black-and-white and was surprised to learn they swear by the Epson color inkset with just Epson driver.
Mark
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Mark McCarvill Photography | markmccarvillphoto.etsy.com