I have a head I pulled from my 9900. I thought it was bad, but it was more likely the selector unit (the device that sits on the head and meters the ink). I have the head in a sealed container, with a moist cloth inside to prevent it from drying out. It was pulled 2 weeks ago. I'm open to selling it to you as-is for testing. PM me if interested.
On the topic of "bad heads" - lots has been written about this, and I believed that was my issue as well. The first nozzle check after swapping the head for another used one, and adding a new selector unit was exactly the same as before I pulled the head. In my case there was no black or light black in the nozzle check print. However, as the system started to finally pass some ink, both of those "dead channels" started to fill in.... after about 15 nozzle checks and a couple C1 cleanings it was 100%. The used head I tried was from a 9900 where magenta had dropped out, and their new head didn't fix the problem. Magenta worked fine for me before and after. I have my suspicions that all these "bad heads" are actually "bad dampers" or selector units. The selector is $175 from Compass Micro, 10% the cost of a new head. It's worth the shot, IMO, to start with that. YMMV.
Aaron