External step-up transformer with Io or MP-1

I realize that the Io, MP-1, and many, many other phonostages have enough gain to work with low-output MC cartridges.
I'm not concerned with what is technically possible or with what should sound better in theory; I'd like to know what people have found to sound best in practice. The Manley Steelhead, for instance, can amplify low-output MC cartridges through its MC input but many users prefer to use an SUT and the MM input of the Steelhead.

I'd like to know what people have found to sound best in practice.

Unfortunately, there is no general rule. Some Cartridges sound well without SUT AND are also good sounding when you connect it to such a unit.
And some wake up with it, some not and there is everything in between.
The main problem - IMHO - is to find a good SUT, most are colouring and degrading the Signal. The only one I can bear is the Kondo SFz.
Sometimes it's Magic.
Not always, but sometimes.

Sometimes it's Magic

What you're saying is that in rare instances, when perfectly matched with a particular cartridge, an SUT offers more benefits than just the reduction of noise.
This is what intrigues me.
Have you heard any of the offerings from Auditorium 23?

I've tried the MP-1 both ways- a few years ago some ex-partners of mine were real advocates of the SUT. My experience is that while you can get the overall setup to be quieter, ultimately you find that the phono is not as interesting as before- brighter, less bass impact, less transparent- in effect, more 'digital'. Ick.

So I went back to direct-in. What an improvement! The big deal seems to be making sure you actually have quiet tubes :)

After I got the company back, we introduced the MP-1 MkIII. One of the big improvements is in the phono; vastly improved constant current sources in the phono section cut noise, increased gain and reduced distortion. In effect, there is even less incentive to run a SUT than there was before. I've been using the ZYX Universe and also the Transfiguration Orpheus with no worries.

Thanks Ralph.

My experience is that while you can get the overall setup to be quieter, ultimately you find that the phono is not as interesting as before- brighter, less bass impact, less transparent

These traits are what I would expect to hear from an SUT and, as such, I was surprised to hear people taking about the 'magic' that some of them provide.