Hi
I see you have been troubleshooting a print quality issue with your Officejet 6600 Printer, and I may have found some things you could try.
HP recommends that you use genuine HP ink cartridges. HP cannot guarantee the quality or reliability of non-HP ink cartridges or refilled HP ink cartridges. If you do not use genuine HP ink cartridges, the steps in this post might not resolve the issue you experiencing. You could also see how non-HP supplies compare to refilled or remanufactured cartridges from the Buyers Lab Report.
First, have you tried all the steps in; Fixing Print Quality Problems for the HP Officejet 6600 e-All-in-One and 6700 Premium e-All-in-One (H711) Printer Series?
If the issue continues, please Contact HP Support for further assistance. Fill in the model number and the form to receive a case number.
Please let me know if these steps resolved your issue, or if there is anything else I can do to help.
I look forward to hearing from you!
Thanks,