Again something is wrong or the reason why the printer ended up on Ebay was because there was some fault in it.I bought a Pro 10 on Ebay and had the exact same problems you did. I'd previously had a ip4700 that worked great for about five years before printing darker and darker wasting ink. I wanted to print larger format so did some research and settled on the Pro 10. Out of the box it printed so dark the images were unusable. I couldn't get a print profile to work for more than one image even if another image had the same values. Calibrating for every stinkin' image wasted a huge amount of ink even though my tests were 4"X6". With much effort I was able to put together one show of 15 images which paid for the miserable thing. After that I used it so sparingly I believe the heads clogged and I got an error code that said to take it to a Canon authorized service center or some such. The nearest one from where I live is about 110miles each way, and obviously a behemoth like that can't be shipped for much less than the price of a new one. I used it for a few months and now it's under my bed.
I know changing the head is easy but am I going to dump another $110 or whatever and get a different error code? It's such a piece of garbage even if it works again it wont work again correctly. Its been a couple of years so maybe I'll see what's out there but electronics seem to vary so much from one printer to the next of the same model. I'm even switching my photo gear from Canon to Sony as soon as the AR3 comes out as well. Canon screwed me
If you had purchased a new one with warranty, Canon would have taken care of the problem and you would have a different state of mind today. I can attest that I have owned the Pro-10 for around 4 years now and not experienced any of the problems you encountered but mine came directly from Canon and was new.
Your statements certainly indicate that either you had a workflow issue or was not using OEM ink not a good quality inkset. It certainly sounds like you were using ink that were aftermarket and the aftermarket ink seller had put in black ink into the grey cartridge and the problems you experienced is exactly what would have happened. I have a hunch you were using aftermarket ink with the Pro-10. Were you?
The other thing is that the Pro-10 when used with proper inks is not a clogger. Using Epson type inks in it would GUARANTEE clogging.