Added an SUT...not sure I understood this

The lower the ohm rating on a tap the higher the gain.  As a rule of thumb, you should use the tap that most closely matches the internal resistance of your phono cartridge.

Your tx is 1:10. Take what the input resistance is at the phono preamp - it should be 47k, and divide it by 100 (10 squared). That is 470 ohms if you have 47k. This number should be at least 10 times the internal resistance of the cartridge. I suspect the input resistance is less than 47k and you are pinching the current. Remember ohms law - current is a relationship between voltage and resistance. I have several SUTs and it is worth noting a that even when the correct resistance is applied, the apparent loudness does not follow the voltage. It’s misleading. 2.5mv and 5mv just sound the same in terms of loudness.
 

 If the goal here is to chase down a hum, and if the tx solved this, sounds like a grounding issue. Try reworking the grounds. Ground the TT to the phono only. Try a star ground. Try shielded cables. Try shielded cables with the shield only landed on 1 side and switch the side the shield is landed on between the TT and the phono pre. try to disconnect all grounds and use a loose ground wire to touch the outside of the rca connections. Ground hum finds a path of least resistance to ground and goes there. Usually this is whatever has the largest power transformers. 

@woofhaven1992 

Here is what you need to know -

Your Audiotechnica AT-OC9XML has an internal impedance of 12ohms and voltage output of 0.4mv

Therefore it needs to see a load higher than 100 ohms ( as per specs ).

When you ran the OC9 straight into the SP14 with a 40ohm loading resistor you would have strangled the sound.

Now the AU320 step up.

The 3ohms and 40ohms on the front panel refer to the cartridges internal resistance - NOT the loading.

With the 3ohm input, the gain is 36x which means the cartridge will see a load of 36ohms. The SP14 phono will see 14mv. The sound should be awful because the cartridge at 12ohms cannot drive 36ohms, and the FET based front end of your phono is probably overloading.

With the 40ohm input on the AU320 the gain is 10x which means the cartridge will see a load of 470ohms which is fine, the phono input will see 4mv which is perfect.

Adding loading to transformers can be a minefield, therefore I would suggest you run the AU320 into the SP14 phono at 47k without any additional loading resistors .

What I would suggest is get a pair of 470ohm resistors and try running the OC9 straight into your SP14 loaded at 470ohms. This will give you a direct comparison with and without the step up with the same loading. You might find the SP14 phono with the OC9 at the correct loading may be fine. Of course the additional step uphill add gain if you need it.

A Schiit Mani 2 ($150) with its mc gain setting probably will sound better than the SP14 phono/SUT combination.