Can I use a step up transformer?

Dear Raul,
Your position is quite a theoretical one: "the best SUT is no SUT".
Of course you always have to deal with compromises.
The weak point in matching is - and you are talking about it too - the phono stage.
When you have experimented with so many MCs and SUTs (and also phono pres) as I did and still do my personal conclusion is you need an excellent phono stage like the EMT JPA 66 or the Boulder 2008 avoiding SUTs. The funny thing is that the quality of the resistors you put on the personal cards in the Boulder is decisive for the sound. Such a small little thing!
You don’t need need investing a big sum on a phono stage when e.g. you go for a Manley Chinnok and use the MM input with appropriate SUTs.
You will be surprised what results you will hear and maybe you will change your opinion quickly.
Manually wiring of transformers is the best way you can do. Of course they are using small machines when doing so. 10 years ago I saw it at DaVinci Audio Labs when Peter Brem was still alive. I think in Japan some high quality producers like Hashimoto use it too.

Of course there are pros and cons to both sides of the argument.
I was happy with the inbuilt phono-stage of the Halcro DM10 (which was compared favourably to the Boulder 2008 in Stereophile) and never contemplated an SUT until I had the opportunity to hear the Kondo KSL-SFz in my home system with a variety of cartridges.
Now there was not an overwhelming difference in the overall presentation of the two methodologies (in my case at least) that anyone could say was immediately apparent and it took a great many listens to a variety of music for me to become convinced that the Kondo offered just a smidgeon more transparency on most cartridges.
On the Sony XL-55 (0.2mV) there was the slightest hum audible (only between tracks) and thus I prefer to run that through the active Halcro MC stage.
The Kondo is one of the priciest SUTs available, totally wired with silver and with silver RCA interconnects so perhaps a lesser SUT might not outperform the active Halcro phono- stage?
As with most things in this hobby of ours.....it's hardly advisable to proclaim the superiority of a single universal direction.
You pays your money and makes your choices 😎🎼

Investing in SUTs depends on how good is your phono chain and of course the system at all.
Using high efficieny speakers is an add on in my opinion as well as a good reproduction of high but also low frequencies.
Let me state in a somehow provoking way: it makes no sense using SUTs which allow you to harmonise the sound but also push it in the more stable and deep sound producing way when you have a mediocre table and phono stage. 
The good news is: today not so many real audio afficionados stick to a mediocre phono line and system. Good systems deserve using SUTs! The more excellent MCs you are using the more you understand the importance of SUTs. 
I like my MMs too. I am sure I will never convince someone of SUTs who is running MMs most of his listening life.
There might be some exceptions 🤗

One advantage of using an SUT (which I've never seen mentioned) is the ability to now alter the loading just as one can do with a MM cartridge.
Because the SUT plugs into the MM input of the phonostage, I can dial in a Resistance anywhere from 10K Ohms to 60K Ohms and the cartridge responds in the same way that a MM does.
Now I don't understand the science behind this and hope that others here can explain what is actually happening......but it works a treat on taming an aggressively bright LOMC and conversely, one which may sound slightly recessed or shy?