Added an SUT...not sure I understood this

By the way, when we speak of MM inputs vs MC inputs, those are euphemisms for low phono gain (usually 40 to 50db) vs high phono gain (usually >60db). Also with your SUT set at its 40 ohm mode(1:10 ratio), and into those 100 ohm resistors, the load seen by the cartridge is 1 ohm! 

@lewm is right and so is @dover . At this point IMO the thing to do would be to get someone to remove those 100 Ohm resistors. SUTs are usually designed to drive 47KOhms (although Jensen makes one designed to drive opamps that expects to see 6.8K).

Dear friends: This is what posted the OP:

"" @rauliruegas Yes I have been using the SP14 with no SUT for some time, and the sound is great except for a touch of hum just at the level of the noise floor. I decided to fiddle with this SUT as a way of addressing that problem, ....""

So, he already knows that in that way the QS is excellent but that hum. Nothing is perfect and always with trade-offs but that SUT the OP choosed is a bs of SUT, it's a wrong path not only for that cartridge but for any cartridge in any audio system.

In the other side if that hum is at the system noise floor then it's not really a problem to die for.

R.

There's nothing per se wrong with having those 100 ohm load resistors on an MC input, most of the time.  So, I would not see the point of removing them except if one wanted to load an MC cartridge with a higher resistance.  The problem seems to have been connecting a SUT to a pair of MC inputs in the first place, unless I have missed something.  I finally broke down and looked up the specs of the SP14. It seems to have only an MC or high gain (66db) pair of phono inputs.  The nominal load is said to have been 47K ohms, but the reviewer (in TAS) says his unit was factory loaded with 100 ohms, like the OP's unit.  So, no SUTs allowed or needed here, hum or no hum.  Especially since the linestage section is said to add 20db of additional gain.

@woofhaven1992

Glad we got that mystery solved!

As for the MC vs. MM - perhaps semantics but I wouldn’t consider the SP14 as having an MC stage. An MC stage would have a ~ +20dB headamp or SUT up front *before* the RIAA stage. This SP14 has a 44dB RIAA stage up front, and then a high gain 20dB line stage AFTER this. Maybe on paper that’s fine, but personally I haven’t found good results feeding too low a signal level into an MM RIAA stage and then "making up" for it with line stage gain. This suboptimal arrangement may partially explain why OP liked it with such a low loading impedance (20 ohms, or even less if in parallel with 100 ohms), in an effort to change the sound.

I disagree with ARC’s decision to solder 100 ohm resistors on the phono input. That absolutely kills any flexibility. With 47K fixed input, a user could always get a loading kit (like from DB Systems) if they needed to change loading for MC direct-input.

Also agree with others - a 12 ohm coil like this AT cart should be loaded at LEAST 100 ohms or greater. As long as this 100 ohm resistor is fixed on the phono input, that means absolutely no SUT, and no additional loading should be used. You’ll start losing dBs and modifying frequency response as you move lower.

Personally I'd clip out those 100 ohm resistors and resume using it with the SUT!