Hi Cincy_bob, here we go again...
It seems the perpetual misunderstanding when looking into SUTs, that the 'normal' recommended (experienced) best cart loading into a MC phono-pre (non-SUT) is incorrectly considered with SUTs i.e. 100 -150ohm as you suggest.
This is completely off the wall when using an SUT.
If you care to look-up e.g. Lyra's spec. sheets you will see what I mean. Their recommended values for SUT is about 1/10!
My experience, hands-on, tells me 6ohm x 2.5 = ~ 15ohm! is a good point to start and as I said, about a 1/10 of what you are suggesting.
Also, what Restock mentioned is much more where it's at, also when it comes to impedance matching.
Lewm, what you overlook is the voltage drop going through any SUT. It is not just 0.3 x factor 10 = 3mV and most certainly not when you decide to do some resistive loading to go from 470ohm to say ~ 15ohm.
I think the 1:20 suggestion of Restock looks like it, and then it be better to consider primary loading not to hog the secondary side with too big a resistor i.e. > 22k
My 2cents worth. There are some sites that explain the maths and I'm not going to do it again.
Greetings,