The rated output for the 901 is 0.5mV, although several reviews I've read have suggested that actual output is somewhat higher. One normally uses a step-up transformer for very low output MC cartridges -- down in the 0.25 mV or less range. Cartridges with output of 0.5 mV usually work just fine with a good phono preamp with sufficient gain -- 60 db of gain is more than sufficient.
FWIW, a local high-end dealer that specializes in analog front ends and sells the Shelter line uses several different phono preamps, but does not recommend a step-up transformer.
FWIW, a local high-end dealer that specializes in analog front ends and sells the Shelter line uses several different phono preamps, but does not recommend a step-up transformer.