Are all stand alone phono preamps both MC and MM?
There is a school of thought that designing a lower gain MM only phono stage is optimal and using transformers to deal with the additional gain of an MC cartridge. Given the diversity in characteristics of MCs (both output and loading needs) manufacturers often elect to leave the selection of transformer up to the user. Allied to this is a belief in keeping the phono stage as simple and option free as possible to focus on the purity of the amplification and equalization
That's at least the philosophy of the maker of my phono stage, which is a pretty high end model and I will say that for this end user at least it works well