SUT for AT-OC9XSL?

I have a Bellari in my collection of SUT's. Excellent for around $320!

Rothwell should have one and he uses Lundahl transformers. You might even be able to find one on the pre-owned market at your budget. New might be a little higher but not way out of budget.

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.

I believe my current phono stage is rated at an Input Impedance: 100K ohms
Output Impedance: 600 ohms
Phono Stage Gain & Sensitivity: 70 dB, 5 mV. I am thinking about replacing it with a Hagerman Cornet2, which has only 44db of gain. I appreciate your response, I am still trying to wrap my head around all the science of how this works. 

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. 

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.