I am hoping to pick up a 16/600 in the next few weeks.

Is there a mirror or lens that needs cleaned? That worked a treat with another Laserwriter of mine. Alternatively, have a close look at the drum on the cartridge and see if you have excess toner on it (it should be squeaky clean). I have had more than one reconditioned toner fail on installation. Fortunately, the 98a/98x is very inexpensive.