How important is the cable between the SUT and phono pre?

Dear @billwojo  : You need not only the short cable you can use but the best you own.

Now, I  still own the 340 and is a good SUT if and only if you make some changes on it: first are the input cheap RCA connectors for a decent ones, second that that short and high quality output IC cable be hard wired directly to the inside 340 board and third that all those internal input wires/cables connected to the 340 board and to the input connectors be changed by better quality wires.

Regards and enjoy the MUSIC NOT DISTORTIONS,

R.

Those changes makes/converts a good SUT in an excellent one that competes with any today top SUT.

Regards and enjoy the MUSIC NOT DISTORTIONS,

R.

looking at transformer equivalent circuits I don't see reflected capacitance.  Reflect impedance, reflected leakage inductance, but not capacitance.  Do you have cite for this?

@jpjones3318 Transformers are called that because they transform impedance. They do not isolate it!

So a load on the output of the transformer very much affects the source on the input side. This is part of why the transformer must be loaded correctly. Incorrect loading of the transformer can have other effects too- for example all transformers have inter-winding capacitance. If the transformer is too 'lightly loaded' (output load impedance is way too high) the inter-winding capacitance will begin to affect frequency response, perhaps manifesting as a hump in the frequency response rather than being flat.

Impedance != capacitance.  

MC step-up transformers explained

"The capacitance of the cable connecting an mc step-up transformer to the following phonostage also plays a part, which is why the interconnecting cable should be a low capacitance design and kept as short as is practical."

Hi @imhififan, he's basically explaining a zobel like I mention above, and some generic advice that may or may not fit a circumstance.   However, transformers do not reflect secondary load C to the primary.