SUT for AT-OC9XSL?

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.

@lewm My amps have an input sensitivity of 1.25V,

my line stage is a VTL TL2.5i with 20dB of gain, and I am almost certain now I am going to get the Hagerman with the 48db of gain. Given that my cart out must 0.4mV, I would need a SUT with a 1:12 turns ratio…? 

Staying under 300 may be a tough call. If you look at the vintage step ups from 1970s through 1980s they seem to start around 450 and up. You may want to consider a ned clayton base and buy either cinemag transformers or the OJAS transformers which will work well. The combined total will push you over your 300 dollar price point by 150 to 200 bucks. 

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.

@lewm if that were true, wouldn’t I have to have the volume on the line stage way lower (due to excessive gain) than my other sources? The way I have things set right now, I keep my volume at about the same position as my other sources.