The published Fs of 414-8C and -8E are 30.2 and 30.3, respectively. Assuming 414A was the same (see link below,) and a random sampling of them measures 10 Hz higher, what does that tell us?
1) The suspension is now stiffer due to age,
2) They're not performing to original spec, and,
3) "That Altec Sound" isn't what it used to be.
Do we have reason to expect that goop hardens and spiders stiffen after 40 years?