SUT for AT-OC9XSL?

IF you add a SUT, then your signal voltage would be very high, as you suggest. For example, a 1:10 SUT between the cartridge and phono would result in about 10V signal voltage at the OUTPUT of the linestage, not at the input, so yes, you would have a lot of gain going into the linestage and even much more coming out of it. That's why I suggested you try going without the SUT. You probably do not need it.

0.4mV from the cartridge results in 4mV going into the phono via a 1:10 SUT, which results in 10V signal into the amplifier (4mV X 2500), potentially with attenuator wide open.  So yes, you would use a lot of attenuation.

Can you be more specific? I am not sure what you are trying to convey. For example, "way lower" for what? Since your CDP or whatever other high level sources you use is likely to put out at least 1V in signal voltage, generally up to 2V, I would guess that your linestage volume control is mostly attenuating those high level signals. Whereas with the 48db phono driving the 20db linestage, and if you like your music loud, I would guess you would have to turn up the volume control more than for your high level sources. Have you actually listened yet to the 48db Hagerman with no SUT?

With no SUT, you have a net of 68db gain, which means the cartridge output will be multiplied by -2500X. If your cartridge makes 0.4mV, the output to the amplifier from the linestage will be about 1.0V. Adding a SUT is overkill. Try without.

A phono stage with 70db gain does not need any SUT to amplify your cartridge; I say this without knowing precisely the voltage output of your cartridge, but just about any LOMC with output 0.2mV or higher will be more than adequately served by a 70db phono stage.  When you switch to the Hagerman, then you will need a SUT, whichI guess is the crux of your question. So, what linestage are you using, and what is its gain through its line level inputs?  You have to add the linestage gain (for an active linestage, the gain is typically from 0db to as much as 20db additional) to the phono gain in order to calculate the total additional gain needed via the SUT.  There are tables and calculators on line that actually make these calculations easy once you have the basic data at hand to plug into the calculator. The idea is that you want an MM phono stage to see an input of around 5mV, because that is the typical output of a high output MM or MI cartridge.  Thus if your AT cartridge makes 0.5mV, a 1:10 SUT is fine.  The 44db gain of the Hagerman will amplify its intput by approximately 160X. That will turn 5mV into ~800mV.  That, even without the added linestage gain, would probably be enough to drive most amplifiers to near full output. The linestage gain will provide cruising space.  So, think of your system as a chain of amplifiers where you want the final amplifier to be able to be driven to full output. You could start with the input sensitivity of the amplifier (what signal voltage is needed to drive it to full output).  Then work backwards to the cartridge.  It's really quite simple but some like the hocus pocus aspect.

You are leaving an important parameter out of the equation. What is the gain of your phono stage? What is the output of your cartridge? If you can make do with a baseline step up transformer that has a one to 10 gain ratio, then there are a myriad of choices that work with your cartridge. A 1:10 SUT into the typical 47K ohm input impedance of an MM phono stage will result in your cartridge seeing a 470 ohm impedance, which is excellent . Your cartridge will even be ok with a 1:20 SUT, but only if the gain parameters suggest you need it. Perhaps you’re confusing the use of a SUT with issues related to using a “trans impedance” phono. That’s a whole different can of worms. With certain transimpedance stages, the 12 ohm internal resistance of your cartridge might not be optimal.