Listen for yourself....

www.analogplanet.com/content/should-you-buy-mm-only-phono-preamp-if-you-are-using-mm-cartridge#2ZMxsjszoWzUU46J.97

DarkLantern blog - darklanternforowen.wordpress.com

As it happens I have put this very question to a manufacturer: They made only two products - A MC only phono stage and a MM only phono stage. Here is an edited Question and Answer:

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Q
Option One: Benedict Hothead MC
Option Two: Benedict Hothead MM with a Step Up Transformer.
Do you have any view on what may be better, or has any user feedback touched on this point?

A
We would always advise the use HotHead C with moving coil cartridges as it was specifically designed to meet the high gain demands at the lowest possible noise level. It is possibly the best specification MC stage around.
Using a SUT / Moving magnet stage will offer comparatively more noise, risk hum pickup and possibly alter the RIAA de-emphasis.

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I was kind of hoping for that sort of answer as the two units cost the same! I am seriously happy with my Hothead C and would buy another one if I could - alas Benedict Audio appear to have fallen off the radar.

Hi @sl1210 - a SUT is a passive device & does not of course fundamentally introduce any noise of its own.
However, SUTs in use, can be subject to hum pick-up if not Grounded appropriately.

RIAA de-emphasis is referring to HF de-emphasis at 2120Hz/75us & the thought that a transformer may exhibit non-linear or curtailed response above there.
A good audio TX *should* of course provide flat & reasonably extended response.

The main thing is that it works good in your set-up & a one-box solution MC phono should be less potentially problematic.

I'd be keen to see a pic of your Benedict phono in your set-up, a fairly rare device I'd imagine

DarkLantern blog - darklanternforowen.wordpress.com

I will post a couple of pics soon

Mr Fremer concludes his reader test-survey with conclusions:
www.analogplanet.com/content/perhaps-you-should-buy-mm-only-phono-preamp-if-you-are-using-mm-cartridge

DarkLantern blog - darklanternforowen.wordpress.com

It would depend on the circuit involved. Some MM-MC phono stages, such as those in integrated amps, have a switch that just alters the gain of the same circuit stage. They are usually best at MM performance with MC being an afterthought. In that case a separate MC stepup device or SUT would be better in order to use it with an MC cartridge. It'd likely be fine for MM though.
On the other hand a phono stage that's been made specifically for MC would have been optimised for that type of cartridge.
So I think generally, yes. MM phono stages are best for MM and MC-only good for MC.
The beauty of having an MM-only stage is that you can use your own choice of MC stepup device in front of it for MC duty.
I make both an MM stage and an MC stage with the Sonic Imagery Labs discrete opamps.

AMR-iFi R&D