Audio Note stuff is typically very good but expensive
I have heard a number of systems with Audio Note SUT, including my own, and they are very good. Given that you have an all Audio Note set up for electronics, it might be worth the price premium to stick with their transformers.
The thing is, there are plenty of quite good transformers on the market, and most of them will sound pretty good, if not quite the same as the Audio Notes. Almost any decent transformer matched to the particular cartridge and gain requirements of the system will sound nice. I sort of agree with Art Dudley of Stereophile who said that even when the step-up transformer is not a perfect match, it will usually sound good; he said that he has never found a step-up transformer he didn't like.
With the ART9, you don't need much additional voltage gain from the SUT in order to drive an MM phono stage, maybe 10X would be overkill, in fact. So, if you're interested in doing this in a scientific and cost-effective manner, so far as that is possible, you might try some of the well regarded but less expensive SUTs before spending the big bucks on an Audio Note. However, if cost is no object, there is no doubt that the AN SUTs are "very good". You can't go wrong, as they say. I don't know why you would need reassurance from any of us on that score.
I personally think everything they make is "very good" and overpriced.
Dear @pani : " They have a version which matches my cart but I just do not know if the prices are justifiable. "
LOMC cartridges are not sensitive to SUT impedances due that the internal cartridge resistance is really low as is the inernal cartridge inductance.
The one that can react to those SUT impedancs is the phono stage and this situation is not a good one to have but almost all the time is happening and will happen with your Audio Note phono stage it does not matters which SUT is your choice.
Audio Note SUts are good perhaps only because the transformers are coiled with silver wire and nothing more but exist other silver wire SUTs that are good too and many not silver wired that are very good too.
Never mind, what you need to match is only the SUT's gain need it for the cartridge output level and phono stage gain.
Regards and enjoy the music,
I am not sure that ALL Audio Note SUTs use silver-wired transformers. For those, you pay even bigger big bucks.
A SUT or any other transformer, has no inherent impedance at all. Transformers merely reflect the impedances of whatever is attached to the primaries and secondaries, as the square of the turns ratio. Thus, since most MM phono stages will present a 47K ohm resistance on the secondary side of the SUT, for a SUT that gives a 1:10 voltage gain, the cartridge will "see" 470 ohms (47K divided by 100).
Like Raul, I feel that this business about "matching" the cartridge to the SUT is a bit over-stated. You want to use a SUT that provides the voltage gain needed to drive your phono stage, and not more or less voltage gain than what is needed. After that, it is a more complex subject to talk about improving the linearity of the response by using Zobel networks, etc, around the output of the SUT. But that's not something built into any SUT I know about. Like I wrote above, in this case, 1:10 would be more than "adequate". 1:5 might even work well for the ART9, but I don't know the gain of the AN phono stage.