Step up transformer or upgrade phono preamp?

I have three MC phono setups:
MAGI Phonomenal phono pre + Live! MC-10 MC SUT
MAGI Phonomenal phono pre + Denon HA-1000
Allnic H1202

MAGI has two phono inputs.
MAGI + LIve! is just as good as Allnic.  Denon sounds a bit inferior to the other two.
If I were you, I would sell Vincent pho 701 and get Allnic.

I did a little homework, reluctantly.  Your phono stage provides 40db of gain in MM mode and 60db of gain in MC mode.  You would need to use the MM inputs with an external SUT. That means you would need a SUT with a turns ratio of 1:10, just to get a total of 60db of gain via your MM inputs, which you already have via your MC inputs. (A 10X voltage gain is equal to the 20db difference between the two phono sections.)  You would have to find a SUT with a higher turns ratio in order to benefit from any SUT in terms of gain.  Moreover, I see that your phono stage uses just one tube, the 12AU7, which means it must use solid state devices for gain and may even have a bult-in 1:10 SUT at its MC inputs.

My experience is that a Phonostage will not excel into a New Dimension by choosing Add On Partnering Ancillaries for the MM Input .
Attaching a SUT to a MM Input will increase the Options on how to replay a Vinyl Album with a MC Cartridge Only.

The SUT will present a different Sound Quality, and in many cases this is the option that users like to have a consideration and make a decision about.
From  my experiences undertaken, usually a SUT is able to supply a further flavour to the capabilities of the Phonostage.

I am yet to experience a Phonostage that has undergone a Transformation from the attachment of a SUT, where it has been perceived as falling short in the Performance, to one that is seen to have now received a Ancillary that is now perceived as the fix all attachment.

A not too impressive Phon' will pretty much maintain that perception, albeit, with maybe a slightly different presentation.
A Impressive Phon' will usually maintain the perception of the qualities on offer, but again, with a slightly different presentation.
 
Adding a Different SUT's will also offer different perceptions to the Sound on offer.
I am fortunate to be able trial a variety of SUT's along side my owned models.
The SUT's Interface with a given Phonostage can vary considerably, so picking a SUT is a game of Trial, Selection or Rejection.

One thing that is very noticeable is the SUT can impact on the Sound Quality by adding a Richness and Perception of Weight.
Different Models will offer the perception of added Richness and Weight at differing perceivable levels.

Again drawing on my own experiences,  I prefer a SUT's influence on a SS Phon' over a Built in MC Stage, this is limited to the SS Phon's I have owned and had demonstrations off, I can't hear them all.

With a Valve Input / Output Stage Phon',  I can lean either way, a SUT with a slight Trait of Richness or a Head Amp compete closely and I can become very comfortable with both in use quite quickly.
The Head Amp's that I am able to use, have a Air and Lucidness in their presentation, they float that little bit more and are more ethereal.
It is not the antithesis of the SUT, but can show the SUT to have a bit of a Mud on their Wellington Boots 

Very recently when using a owned Valve Hybrid Phon', I was quite happy with the SUT's and Head Amp's being used, these were more desirable than the built in MC Stage.
During the Trial the desire developed to keep the Head Amp over the SUT as the Ancillary for the extended listening to the Phon'. 

What is most noticeable when swapping in out the Ancillaries and Phon's is how on one Phon' with one Ancillary, the Bass can recedes, and become quite a unwanted presentation.
A change of Ancillary and the Bass can becomes very noticeable as a comparison.
When using a different Phon' preconceived ideas are short lived as a Ancillary that is suspected to perform in a Certain way and not be the most impressive can become the Ancillary that is on Par with others that are offering most attractive impressions.

I am not a Bass orientated person, but when it becomes unusual and unfamiliar it is a detractor, and not desired to be maintained in such a presentation.

For the OP, I feel that Loans of Ancillaries that work with your MC are the best method to commence with.
Compare a Ancillary in on the MM Input to the Phon's Built In MC Stage.
That will allow for Comparison Numero Uno to be undertaken.
Leading on from this and few additional Loans, it is possible to commence to create a Short List and discover a Ancillary that produces a Interface that will be most suitable for your unique preferences. 

Trying to arrange a Loan might require a Deposit, or a Favour from a not too distant fellow enthusiast.
Usually in my experience a offer of Social get together and a little Hospitality will produce a few SUT users keen to assist.  

My Proposal will definitely aid you in the Search that is                        'constantly looking for better'.

It might also help you to assess the real value of your Phon' within your Vinyl Chain, and reconsider the thought of it being a Weak Link.    
   

The only thing i will state is, does your pre amp or integrated amplifier have a phono stage, if it does not, then i would invest in a better phono stage, if it does have a mm phono stage than a step up transformer would be the way to go.

There is a change to the Sound being presented that is most likely to take place when a MM Input is coupled to a Step Up Ancillary.

The more varieties of Ancillaries being trialled are likely to produce a variety of changes to the Sound being presented.

The Crux is, are any of the Perceived Changes to the Sound being Presented, an improvement on the Phon's performance or is the Phon'  remaining very similar, with a few extra flavours being added ?

There is not much of a requirement needed to begin to experience the affects of a Permutation of a Phon' and Step Up Ancillary