I've only heard a phono stage with an active headamp.
In your experience what are the sound qualities that a passive step up transformer brings to a phono stage?
A Moving coil cartridge has a very small 'Voltage' output but a very large 'Current' output. So in my opinion what you want to do is work with the large current output rather than the very low voltage output of the MC to optimize the noise floor of your phono system.
So the passive step-up transformer "transforms" the high current output from the cartridge to high voltage which is required by your MM Phono stage. The quality of the transformer is critical of course and the cost is generally much higher than an active headamp but in my opinion it offers the best performance.
If you look at high performance MC cartridge manufactures they generally offer step-up transformers as I believe they are cognizant of the advantages as well.