I am just back to the office after being out with a sick baby for the past few days, and just had a chance to review your thread and nozzle checks.
Your nozzle checks both show missing and mis-firing nozzles in the cyan/shade #2 channels, the black channels both look fine. I have attached your two nozzle checks below, which I have labeled to show the issues I'm referring to. With Piezography, shade 2 prints in the dark shadows, so missing or mis-firing nozzles in the cyan/shade #2 channel can cause banding issues in the dark shadows.
I also think your ink output is likely incorrect density, which happens when carts are not agitated and the printer isn't used regularly. As per our instructions, carts should be regularly agitated every 1-2 weeks, and printers should be used at least once a week to keep pigments in suspension, and avoid settled pigment, which can cause clogging/flow issues, as well as incorrect output density.
Due to infrequent use of the printers, not shaking ink carts, and not cleaning/maintaining the printers, I recommend you shake all the ink carts to get pigment in suspension, and top off any that are less than 1/2 full (always shake ink bottles before refilling carts), then do one Initial Fill Cycle thru the printer's control panel to flush settled ink from the lines, and get good/correct density ink from the shaken/refilled carts to the print head. After shaking carts and doing the initial fill cycle, follow our instructions to manually clean the printer's capping station, wiper blade, flushing box and bottom of the print head. Instructions on how to do an Initial Fill Cycle can be found here: http://www.inkjetmall.com/tech/content.php?151-Initial-Fill-Procedures-for-Epson-Pro-Model-Printers, and instructions for manually cleaning your printer's internal parts, as well as general maintenance information can be found here: http://www.inkjetmall.com/tech/content.php?133-Printer-Cleaning-and-Preventative-Maintenance
When I came to set up your printers with Piezography a few years ago, I remember you had several printers, including 7800/9800, 7880/9880 and 4800/4880. If you don't have enough demand to keep all the printers set up with Piezography ink, and the printers are not being used on a regular basis, I recommend installing the set of PiezoFlush carts you already have, and do an Initial Fill cycle to purge ink and fill the internal ink systems with flush for safe storage. This way, you can focus printing and and keep maintenance to one machine, which the others sit safely with flush fluid until you're ready to use them. It is very bad to let a printer sit unused with pigment inks installed, as all pigment inks will settle (which cause density shift, as well as much faster than normal build up of pigment particles in the dampers), and pigment inks dry solid, which can lead to stubborn or unrepairable clogs in the print head micro-channels. In addition to regular manual cleaning, several parts in the printers should be replaced every few years for regular maintenance, to keep the printer clean and working well (and lasting as long as possible). Dampers are in-line ink filters that protect the print head from particles that could cause clogging, and the set (8) of dampers should be replaced every 1-3 years. Along with dampers, the wiper blade, capping station and flushing box should also be replaced about every 3 years (as well as being cleaned on a regular basis).
I hope this helps. Please let me know if you have questions, your results after doing what I suggest, or there's anything else I can help you with.
Best regards~ Dana