Brian,
Early magnetic carts., like GE variable reluctance, are not designed to work into the, now ubiquitous, 47 KOhm load. GE developed a circuit that uses the inductance of the cart. as part of the EQ network. I've uploaded a Fisher implementation of the GE design.
I agree with your desire to have adjustable EQ. "78" RPM disks were cut with several different curves. Also, very early mono LPs can have EQ somewhat different than the curve now known as RIAA.
BTW, very early, pre electric, "shellac" may well sound best on a horn loaded mechanical reproducer.
Eli D.
