SUT for AT-OC9XSL?

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?

Assuming the signal of the turntable (because of the SUT) is much higher than it should be as it enters the line stage, the amount of gain (volume) you would use on the line stage preamp would be lower than normal correct? Let’s say my CDP inputs a signal of 1-2V, wouldn’t the turntable signal of the turntable be markedly higher than that (hence what makes it excessive gain), causing me to have to turn the gain (volume) on the line stage way down when on my turntable input to compensate for the additional gain that happens upstream of the line-stage? in other words, the the volume controls gain on the lineage right? So if the turntable signal is much higher than 1-2V (because of the gain in the SUT and Phono-pre combined) then that source would need less gain (a lower volume position) in the preamp line stage than other sources which are not amplified much before the lineage?

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.