Trevor Wilson wrote:

I've got a MF LPS phono amplifier rated at 100 ohms input impedence
for the MC bit, and an AT OC9 cartridge rated at 20 ohms load

Most MC carts are designed, specifically, to be terminated with
a low(ish) load impedance. Unless so terminated, frequency response,
dynamics and distortion will all suffer.

So put a 27 ohm resistor in parallel with the input.

If the cartridge wants some load capacitance, add a bit in parallel.
The amp's input capacitance, that of the cable and your extra capacitor
should add up to the cartridge's recommended load capacitance.

--
Eiron.

This advice is worth what you paid for it.