[old_tv_nut]

I don't have enough direct experience with this particular chassis to point at a particular component and say "look at that." The curves are close to
the manual except more peaked in the center, so actually may be within normal range, especially since you see differences depending on the cable.

One experiment you could try: Since A9, A10, and A11 have their tuned frequency specified on the schematic, simply tune each one back and forth a bit to see if each is really producing maximum response at the specified frequency. If they are, I would say there's no more to be done.