I am a huge fan of SUTs. I agree with mulveling's first suggestion. Forget about using the PL100's MC stage. Go through an SUT into the MM section. I know that you spent a bunch of money upgrading the MC tubes and don't want them to go to waste. However, I find that the simpler that you can keep the signal path, for the most part, is for the better. The MC tubes may give it color, but they will also add noise. Also, you are entering a problematic area trying to interface an SUT with the MC input. I would be very worried about loading down your cartridge and dropping your input voltage right out of the gate. The MM stage will have the standard 47KΩ input impedance. With a simple SUT. Which I also believe in. The reflective impedance that your cartridge will see is calculated by taking the output impedance of the secondary and dividing that by the square of the winding ratio. So for a simple example. A 1:10 SUT into a 47KΩ load will be 47,000/100 = 470Ω. That is the highest reflective input impedance that you can achieve. However, you can lower that value by putting a resistor across the secondary in parallel with the 47K. It is possible to load the primary input, however that makes things much more complicated. You will cause ringing in your output waveform, unless you design it correctly to stop that from happening. And I fail to see the point of having a fixed input impedance, with all of the issues that that implies, when you can achieve the same result by adding a resistor across the secondary in parallel with the 47KΩ impedance of the MM input. With no complications what-so-ever. If your cartridge requires a higher impedance load than 470Ω. You must reduce the winding ratio. Thus sacrificing gain. But the specs of your cartridge give you a lot of range. I doubt that you will need more than 470Ω. More than likely less.
If you are interested in experimenting with using a SUT. I would bypass the active MC tube section altogether. That is basically what a SUT is for. It is a passive gain device that gives a cleaner simpler signal path. That is why they are used. I find them to be much quieter, and they give a much more open sound to the music. People will tell you that they are difficult because they are prone to EMF humming and must be placed in the right spot. I haven't found that to be much of an issue. However, the interconnects between the SUT and the MM inputs should be a low capacitance cable as short as is reasonable for your situation. Because the MM input is very sensitive to capacitance. I would go with that experiment. If you find that you prefer the active tube step up. You can always use that. But trying to interface the two, is problematic at best. And probably will not give you what you are looking for. There is a company that will custom make you whatever SUT fits your needs for a very reasonable price. Even cheaper if you are willing to assemble it yourself. They use Lundahl transformers. Check them out. https://www.erhard-audio.com They came highly recommended by Peter Lederman of Soundsmith cartridges. I believe mine cost me under $400 assembled. And it sounds great! It is a simple task to swap out resistors to fine tune the loading of your cartridge. I will never go back to active MC step ups. Even if they are tubes. I am using a SUT into an Icon MM tube phono pre, into a PrimaLuna EVO 300 integrated and I love the synergy. I am sure that the MM section of the PL100 will give you all of the tube sound that you are looking for. I believe that it also uses a tube rectifier. An SUT will give you a nice quiet open soundstage that I think that you may appreciate. And you can always use the MC stage in the PL100 if you find that you prefer that.
