rdk, where did you get the 50mV spec for phono overload? Since the Steelhead has both MC and MM inputs, and since MM inputs generally tolerate much higher signal voltages than MC inputs, there ought to be two different specs for phono overload, one for MC inputs and one for MM. The output from any SUT ought to be connected to MM inputs, and the gain set for the minimum 50db.
By the way , using a 0.5mV cartridge with a SUT that has a 1:20 gain factor into an MM stage that provides 50db gain is way overkill so far as total phono gain. Most MM stages proved 38 to 45db gain, typically 40db. 1:10 ought to be more than sufficient, with that cartridge into the Steelhead’s MM stage.