"An SUT (Some call them Head Amps) are needed if your phono stage doesn't have enough gain to deal with a low output cartridge."
This may be pedantic, mb1, but I think SUT is specifically a passive step-up using transformers (hence the "T" in SUT), while a head-amp uses active electronics to boost the signal; it needs a power supply, an SUT doesn't.
Their merits are debated. SUTs are 'pure' — head-amps add distortion.
But SUTs pick up hum — head amps don't. But head-amps are noisy — SUTs are dead-silent. Some head-amps use batteries to solve the noise issue. My head-amp plugs into the wall, but I improvised a battery supply (easy, it only needs 12V). Frankly, I hear no difference, no noise either way — but I learned long ago my ears aren't golden... and turning 70 didn't improve them.
I have both SUT and head-amp. I usually decide which to use depending on cartridge, trial and error to see which match I prefer.
But I have a new principle — I use my head-amp when using a passive line preamp to drive my power amp, because passives can lack dynamics and 'flatten' the music — the head-amp, being active, restores dynamics: I can clearly hear more rhythmic drive and sonic contrasts.
But when using a normal active line preamp, I use the SUT, because I'm not lacking dynamics. I get the SUT's purity, without the trade-offs.
Maybe just my latest meaningless fad, but so far seems to work for me.
Another virtue of head-amps is they usually offer more adjustments (impedance, even capacitance) than SUTs, to cater to different LOMCs' requirements.
Hope this makes sense...