Nice to read that ET-100 SUT sounds good, I often recommend it based on it’s features: PASS for MM, 3 optional impedances/transformer windings, and 3 tonearm inputs front selectable, but never heard one.
Often available on hifishark, here’s one on eBay for $370. free shipping, 30 day returns
Like you, I was new to MC and SUT’s, I chose Fidelity Research FRT-4, similar features, usually cost more than the Entre, also 30 day return
SUT’s are Passive, no power needed, and actually are designed to deliver an unchanged but amplified signal by one of the SUT transformer’s windings to your MM Phono Stage. Each SUT winding has an x factor of amplification and resultant impedance load.
Your Ortofon SPU MC cartridge’s signal strength is low 0.2mv, it would benefit by an x factor about 20, i.e. boosted to around 4.0 mv.
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Something is unclear.
McIntosh C27 has only MM phono Inputs (2) (no MC capability), The MM input/the C27 phono stage perform the RIAA Equalization. Those MM Phono inputs are sensitive to minimum 2.0 mv. 10 times your cartridge’s signal strength (see pg 11)
SUTs Amplify, but do NOT modify the signal, the boosted signal still needs RIAA EQ.
Either a separate MC Phono Stage with RIAA Eq. (you did not mention),
OR, your C27’s MM Phono’s RIAA EQ. A separate Phono Stage optionally may/can perform RIAA EQ, in which case you use a LINE Level Input on a preamp/integrated.
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Is it possible you were running your MC cartridge’s 0.2 mv signal to the C27’s MM Phono EQ Input? If so, no wonder it did not sound great, and your Preamp’s Volume Control and Amp’s Volume would have to be near maximum.
IF you are now running the SUT’s output to the C27’s MM Phono Input, then the C27 is receiving enough signal strength to do it’s job well, it is doing the RIAA EQ, and the sound character of the C27 should be the same, but much improved by the 'strong enough' signal strength, volume controls down into the range where they are much quieter.
IF you are running your SUT’s output to one of the C27’s Line inputs, then RIAA is NOT happening, thus the signal to the amp would have it’s highs boosted and it’s bass cut, definitely wrong.
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btw, I had a McIntosh SS C28 Preamp, I DID NOT like it’s MM Phono Input’s sound, actually preferred the optional Phono EQ built into the Audio Technica AT120 TT, then into Line Level. Even a $14. Pyle MM Phono EQ to Line Level sounded better.