I have reviewed the attachment showing your gradient/21 step strip output, and have reviewed many prints I have made with Selenium inks on Type 5 paper with GO for comparison. After examining my test prints, I verified the yellow cast that's visible in your print is not normal, nor is what appears to be a magenta/reddish cast in the upper mid-tones, and harsh transition jump between the 45-50% range. I know the 38xx, x900 and x890 printers have a thin ink line labyrinth before the dampers, which could be another place pigment gets trapped and seeps out slowly over time- though, I would think all traces of color would have been purged after using PiezoFlush, then installing Piezography inks... In my experience, four Power Clean Cycles is sufficient to flush ink from a 38xx printer, so after ten, I can't believe there would be any ink left in the lines, damper or head. When you flushed the ink lines, did you also clean the capping station, wiper blade and bottom of the print head to remove color ink build up from those areas as well? If not, I suggest doing so by following our instructions and video tutorial, which you can find here: http://www.inkjetmall.com/tech/content.php?133-Printer-Preventative-Maintenance
Are you reusing your waste ink tanks? If not- it's something that will help save $ and waste, it's very easy by following our instructions and using our 38xx waste tank chip resetter
Please let me know if you have questions, your results after cleaning, or if there's anything further I can help you with.
Best regards~ Dana