You want to be able to feel the horses ribs easily but not see them. If you can't feel a horses ribs you are in serious trouble and the horse is at serious risk of a whole range of health issues.
I would not recommend COB- it stands for Corn, Oats, and Barely. All of those are VERY high in starch, which is a very un-healthy diet for horses. The high starch can cause colic, founders, ulcers, tying-up, hyperactivity, and diarrhea- just to name a few.
I have not fed Fiber max or proform, so no help with that.
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown