I set up an Epson 7900 several months ago with a Warm Neutral ink set and have fought with a few tonal range problems in the darker/shadow portions. One thing I noticed is that shade #2 REALLY needs to be agitated or it comes out a faded blueish tone. Agitated more so than the other shades. Now, this isn't my problem per se... but it is what I thought was causing me some extended grief.
Fast-forward to a few weeks ago when I set up a second PZ printer. This time I installed a custom Neutral set with WM#1 and N#2-#7 and right off the bat this new setup produces MUCH better prints. So, puzzled by the difference in quality I started printing some test charts on both printers and comparing the results.
I think I found out what might be going on. It appears that my WN set has the wrong ink for shade #4. When printing via QTR in calibration mode I can see that my WN#3 and WM#4 are different colors, but they appear to be the same level of darkness (more or less...) when I did the same with my N#3 and N#4 I could see a more dramatic difference in the black level. It's hard to say if WN#3 is too light or WN#4 is too dark. If I were to rank them from dark to light it would be N#3, WN#3, WN#4, N#4. I tried to attach a photo but the forum wouldn't take the upload.
My ink bottles have the following lot numbers:
WM#2: PZK7-WN-700-2 Batch #: 980417 Exp Date: 01/2016
WM#3: PZK7-WN-700-3 Batch #: 980417 Exp Date: 11/2015
WM#4: PZK7-WN-700-4 Batch #: 980416 Exp Date: 11/2015