My largest concern here is that it sounds like it is the optimal to
build the sut to suit one cartridge so when/ if you do the cartridge
square dance ideally you would need change sut too?
No. You simply have to load it correctly for that cartridge.
I still don't believe you :-)
That's a bit of a surprise- your response on AudioAsylum lead me to believe otherwise. I can point you to a number of other examples should you need more proof.
A SUT or line level transformer driving a gate/grid of an active device
is a different situation where any load or network required is a
function of a compromised transformer design and not part of the
transformers "job description"
SUT's typically drive the input of a phono section, where the input standard is 47Kohms. In this situation, the inductance of the cartridge and the capacitance of the interconnect cable form a resonant circuit. This resonance might have to be snubbed else it can cause troubles for the preamp, if the preamp has poor RFI immunity. That's what the loading resistor is for. However, driving a SUT with its additional inductance this peak is a bit different. If you don't deal with it it might cause the transformer's response to not be flat- depends on if the transformer is designed specifically for a certain cartridge, but if the transformer is mounted in the preamp as is the case with this thread where the transformer is expected to work with *any* cartridge then loading should be set up.
I have found that when operating as balanced care must be taken with
the cable layout to adhere to the balanced standard and simply attaching
an XLR connector to an existing tonearm wire is hit or miss when it
comes to hum pickup.
Actually you treat it like any other balanced source. The '+' and '-' outputs of the cartridge travel in a shielded twisted pair, the shield is the tone arm ground (pin 3 on DIN connections, pin 1 on XLR connections). If you set it up this way (assuming that the arm itself is actually connected to the ground) there will be no hum whatsoever.