Why does my system now have shrill top end.

Proposition: There are less Morphic fields 👻 👻 👻 floating around late at night. And early in the morning. Especially Sunday morning. Morphic fields are frequently misinterpreted as "power grid activity." You should notice excellent sound quality issuing forth from your system during those times, generally speaking. Very weather dependent, too, low pressure systems bad ⛈ , high pressure systems good 🌞. It’s doesn’t take a rocket scientist 🚀 to realize there are a great many variables involved, certainly not (repeat not) only one or two.

OCOS cables are pretty old ( around 1997 or so).  If I remember correctly, they were designed with OCOS standard terminators (Red), but when used with dome tweeters or a speaker with a rising impedance curve, they needed to have the HFC (Black - nobel) network installed on the speaker end in place of the Red ones.  They also came in VERY specific lengths (9'10"/13'/16'6",etc) which I have to assume was a performance benefit or to correct a design/engineering issue.  I would try and borrow some newer speaker cables and hear if that "fixes" the problem.  Those OCOS cables might have oxidized over the years and/or the connectors might have issues.  I also suspect that the Black HFC needs to be used for the Endeavor speakers.

Tis probably best to think of snake oil as the lubricating liquid that makes the engine go. It also separates the MidFi from the High End, you know, just judging from the responses here. No offense if MidFi is your thing. Everyone has to be somewhere.