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

You have multiple cables. In the time spent asking for guesses here you could hook them up and know. Then let us know. Why not try them all, let us know what you tried, how it sounded?

@billwojo

I am probably not the kind of expert you need here. But look at this page which I found very interesting:

http://www.hagtech.com/loading.html

The first section deals specifically with loading MM cartridges with capacitance. However, the formula Fr = 1 / (2*PI*SQRT(L * C)) works equally for MC and MM cartridges. This formula defines the cart’s resonant frequency based on its inherent inductance and the downstream (cable) capacitance. The resonance is often accompanied by a sharp peak and followed by rolloff. Generally you want this frequency as high as possible, well out of the audible band (> 20kHz). This generally requires low capacitance OR low inductance (as per the formula). MC cartridges get the low inductance (that’s the L in the formula) for "free". Normally this is enough that they need not concern themselves with phono cable capacitance. HOWEVER with a SUT reflecting the output cable’s capacitance times the square of its turns ratio (look this up elsewhere), capacitance could suddenly become large enough to drop the resonant frequency back near or into audible range and become a concern. For example: say you (unwisely) choose a fat audiophile interconnect run with 200 pF capacitance. And your SUT is a 30x. 30 * 30 * 200pF = 180,000 pF. Wow. Say your MC has an inductance of 1mH. Using the calculator in that Hagerman page I linked, the resonance is now at 3,800 Hz. You’re gonna have a bad time! Choose a cable with 40pF and now it’s at 26,000 Hz. Much better!

@jasonbourne52, it's not quite that simple for MM as increasing capacitance will also move the resonant frequency down, depending on the electrical characteristics of the generator, of course.  This is why on a cartridge like the AT150MLX, if you want flat response, the cartridge cannot see more than 150pF.  Also why that cartridge has such a maligned reputation for exaggerated HF response.

@mulveling, 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?

@billwojo, capacitance on the secondary with load R will form a zobel, which can tune the response of the transformer.  For best performance every SUT should be tuned this way, but as it is cartridge, transformer, and upstream phono stage dependent, it's not practical for the average person.  Just know that changing C on the secondary can have an audible effect, but it's to do with the transformer response itself.  

At the bottom of this page is a tech note from Morgan Jones: https://www.sowter.co.uk/specs/8055.htm

@billwojo , The single most important factor is to keep the cable as short as possible. I would use a high bandwidth balanced cable like Canare Da206. 

I use Sowter transformers which I mounted inside the phono stage with a switch to go back and forth between MM and MC. Doing this depends on how much real estate is available inside the phono stage. The transformers are very small. 

https://www.sowter.co.uk/acatalog/E_A_Sowter_Ltd_PHONO_CARTRIDGE_TRANSFORMERS_30.html

Agree with @mijostyn keeping your run out of the SUT super short (0.5m or less) is probably what you should focus on in most cases. 

@jpjones3318 No I don't have a cite for that part. I've seen it mentioned a couple times on forums, but forget where / who. I took that, combined with the info on the Hagerman page (which was fascinating to me), and my own SUT + cable listening tests (more capacitance, longer runs after SUT == BAD) and ran with it haha.