Actually, I think I got it to work. First thing I did was go into Protools 10.3.2 where everything works ok and exported all the I/O settings to files. Then I uninstalled Protools and uninstalled and reinstalled the audio drivers, just in case. After reinstalling 10.3.5 I noted that the I/O wasn't right (really wasn't expecting it to be). I then imported all the I/O settings I saved earlier and everything seemed to work just fine after that. Granted, I only checked a couple of things (it was late) and I need to test it out further, but it's at least working to some degree. I'm fairly confident that all the features of the C600 will work ok now. I'll report further if there are issues.
Being able to export the I/O settings is a brilliant feature. Good idea.
Thanks for the suggestions and help.