Because MM phono stages vary quite a bit in the amount of phono gain they provide, from 39db for the Ypsilon to 50db for the Manley Steelhead, just to name two extreme examples. Then there is the fact that the OP wants to amplify a cartridge that puts out 0.2mV and has an internal resistance of 30 ohms. Those two parameters suggest he is limited to a SUT that provides 10X gain, a 1:10 turns ratio, IF one wants to observe the rule of thumb that says the phono input impedance should be 10X or better in value than the internal resistance of the cartridge. Assuming a 47K phono load resistor, a 1:10 SUT will have the cartridge seeing a 470 ohm load. That’s fine, but as soon as you go to higher gain SUTs (higher turns ratios), the impedance seen by the cartridge will fall way below the ideal, in terms of its ability to drive the phono input. For example, a 1:20 SUT will give a load of slightly more than 100 ohms. A 30-ohm output Z is driving a ~100 ohm input Z. In that case you need enough gain to overcome the fraction of cartridge output that will be lost to ground, because of the relative impedance mismatch. So, in short, seems to me you want an MM phono stage with relatively high gain, as MM stages go, because ideally you don’t want to have to use a SUT with much more than a 1:10 turns ratio. Of course, the OP needs to take into account the additional gain of his linestage, as well, which could ameliorate the potential problem.
I should finally make clear that there is nothing magical about the 1:10 output to input impedance ratio. The fraction of signal voltage that is lost to ground increases linearly as the ratio gets closer to 1:1. Even the 1:10 ratio does not result in perfect efficiency; signal voltage transfer is a bit better than 90% at that ratio.
Once one knows the phono gain and the linestage gain, and perhaps the input sensitivity of the amplifiers, then one can decide whether a 1:10 SUT would be sufficient.
