According to SORAsound, MC's going through step up transformers are sensitive to loading:
"Some general comments, pertaining to more than ZYX phono cartridges
obviously. Assuming you’re not using stepup transformers, the optimum
impedance for a LO (.24mv) ZYX will usually fall somewhere between 50
and 200 ohms. The ideal value will vary from system to system, since it
depends on the electrical characteristics of the tonearm cable, phono
cable and other components. Experimentation in-system is the only way to
determine the optimum impedance for any MC. I’d suggest starting at
around 100 ohms, then moving up or down from there depending on sonic
results.
If you are using step-up transformers, I’ll need to know your step-up
(turns) ratio or gain (in db) to make a recommendation. MC’s are
extremely sensitive to impedance loading when the signal is going into a
step-up, so more experimentation will be required."