I think it is instructive to see the specs of the cartridges Tom (Montesquieu) say's work best with his SUT's and compare them with the specs of the cartridges he is using with his head amp. I am inclined to think that most cartridges with "modern" specs (for example 0.5mV output, ohm impedance like the My Sonic Labs) will work best with a head amp whilst cartridges with "older" specs (like the FR-7 or Io 2 above) work best with SUT's. Most of my cartridges fall into the latter camp but I do have a few of the former and find that whilst they do work with my SUT's they need more experimentation with their loading requirements for best sound.
I think it's mainly down to output ... many older MCs are of very low output, which pretty much always respond better to use with a SUT. (It being just too much to ask a valve in particular, but also most implementations of FETs, to provide sufficient gain to boost them to sensible levels without introducing too much noise.)
SUTs can have their own noise challenges though ... I've learned over time how to set up arm and turntable earthing, as well as appropriate system layout and interconnect selection and dressing, in order to get them dead quiet, but I recall years of difficulty with ground loops or induction hum.
Per the original question - a healthy 0.5 mV at around 1 ohm, that's right in head amp sweet spot, even a tubed one like the HA5000.