Just bumping this thread again. We have continued to struggle with certain gradients on both of our brand new HP printers, with no resolution offered by HP, Onyx, or our reseller to eliminate banding. So we finally downloaded a trial version of Caldera and got it connected to our printers, ran a test print off of our FB machine (just a straight up illustrator gradient) and it looked perfect. No banding at all.
So, if anyone has or is considering buying an HP LX or FB machine, I would highly suggest you go with Caldera instead of Onyx. I'll update this thread if we can find a solution in Onyx, but I have a feeling that pretty soon we will have 2 full versions of Onyx Production house V10 for sale. Stay tuned!