If you have 28db of gain through the phono stage and 30db through the line stage, that's 58db total. If your cartridge has 2.4mv of output, that should be an excellent match. Why do you need a SUT?
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I thnk it's a typo above... .24mv. What about the EAR MC4?
I bought one partly by dealer trail and partly from the comments on Ultra Audio. The reviewer did use a Airy 3...
the Airy 3's output is actually higher than .24mv if industry standard test tracks are used. ZYX uses a less commonly known test LP to measure output, one which results in lower numbers. Multiply by about 1.34 to get an output that's comparable to most other carts... ie, around .32mv.
ZYX's impedance spec of > 100 ohms is when using an active gain stage. That figure is irrelevant for stepups. Many years ago I did the math and then carefully optimized our Bent Audio stepups for Airy 3 and later for UNIverse. It took a lot of trial and error, as the tiniest resistor changes were audible. Calculations are lost in the depths of time but the Bent's ability to easily swap resistors was very helpful.
Even better would be a design that lets you swap resistors on both the primary and secondary windings to control resonance peaks in both the cartridge and the tranny (Zobel network). Truly matching an SUT to a particular LOMC requires a lot of work, not to mention a fair sized pile of resistors.
All that said, one of the best, untuned SUTs I ever heard with a ZYX was a friend's Cotter. If you come across a used one it would be worth buying.