I use a Tube Input / Output Phon' with a few SUT's and loan a Head Amp for periods as a friend has built a variety of guises of these designs.
The Tubes used are Very Early Mullard's, matched on both sides and as a pair.
The selection of the Tubes was from a tube rolling/demonstration experience arranged for me to take part in, that was great fun, as well as indelible for what was learnt about a impact of a tube type on a circuit.
My experiences with trying different SUT's in Comparisons, is that each has its own sonic traits and the ones I have experienced are reasonably easy to detect the changes on offer.
Comparing a SUT to a Head Amp, in my experiences has shown a SUT used in my system, has a hint of richness which in comparison to the Head Amp, I would say the SUT has a colouration, but not a detracting one.
The Head Amp in my system is Transparent and can be tweaked to accentuate certain sections of the frequency range.
My preferred design of the Head Amp to date, is the one that lets a tight Gut Thumping Bass Note to be delivered. This type of device does not have the delicacy that can be an attractor on a SUT.
With this Head Amp the Helicopter in the Track 'We don't need no education' landed in my listening room 😎
Very recently I was invited to receive a demonstration of a Phasemation E 350 which was carried out in a System I know very well and has a Phon' that is produced to a very similar build to my own one, it also has the same Valve Selection as mine.
Both were used with a selection of SUT's and a Phasemation Degauss was carried out between exchanges of SUT's.
The E 350 was totally at home in the company it was up against, and this was with only an hour or so usage prior to being demo'd.
I would really like to hear it with a 500 hour usage time behind it.
I am sure the group at the demo' would recommend the SS Phon' and even encourage somebody to offer it a place in their system.