Hello Char3v,
Welcome to the HP Forums!
I understand you're receiving a print quality issue with your Officejet 8600 plus. I will do my best to assist you! The only thing I can recommend is following this HP document on Fixing Ink Streaks, Faded Prints, and Other Common Print Quality Problems.
In this document please check the "Print Settings" on your computer to make sure everything is correct. Also, how long ago did HP replace the printhead for you? Are you still using the Setup ink cartridges provided with the printhead or brand new ink cartridges?
Looking forward to hearing from you. :smileyhappy: