If you are at 6 weeks there shouldn't be too much feeding left to do. It's a normal part of the life cycle for the larger fans to yellow and die off in late flower. If I were you, I would probably just feed pH'd water from here on out so as to not compound any problems you may already have. Also, you don't want to pound nutrients and make a flush before harvest impossible. Take em another couple weeks if you can, on water, and give em the chop.

Have you looked at your trichromes under a scope yet? That'll tell you how close you are to harvest.

Have you done any transplanting in these plants lives?

How long have they looked the way they do? When did the leaves begin to die off?

Are there one or 2 strains growing?

Just some questions to help out with the analysis.