Oh...another thing...you've got like 400 extra watts in those amps.
At least!

By shrillness, I mean it hurts your ears. But I only hear it on certain songs. What do I need to get to make these old speakers sound new again?
"Driver cry", turn it down, or move up to speakers designed to produce the SPL's your "volume knob" hand craves.

FWIW, Altec "for Heath" components are distinctly and clearly identified, i haven't seen any described here.

tar-filled X-over box sounds like Heathkit
I've never seen a potted Heath XO, do you have a pic?

The green N800-G XO and 803B suggest an "iconic" model, perhaps early enough to have been before the "Valencia" name was given to them, circa 1960-63 or so.

Just my .02.....................