@lewm, thank you for this excellent, informative post on a topic that’s been murky to me.
I have an Einstein The Perfect Match phono preamp and looking to buy a new cartridge. TPM used an unusual circuitry, which, as I understand it, keeps the signal in current until late and doesn’t require changing load impedance. I understand in doing this, with low resistance input, that the gain is dependent on cartridge internal resistance. I am considering purchasing a My Sonic Labs cartridge that has an extremely low 0.6 ohms internal resistance. Does anyone have experience with these and whether or not they will work together?