Added an SUT...not sure I understood this

@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!

@mulveling  : " and resume using it with the SUT! "

With all respect certainly you don't know what you are talking about in this OP specific issue.

R.

mulveling, Perhaps you know this unit much better than I do.  My info was taken from the "specifications" section of a 1990 review of the SP14, published in TAS. Based on those data, it appeared to me that the phono section alone has a gain of 66db and that the linestage per se adds 20db of gain.  But I certainly could be wrong.  This is from a second source:

• Preamp Output: 2V (50V max)
• Frequency Response: 5Hz to 50kHz
• Distortion: 0.01%
• Gain: 66dB (mm), 20dB (line)
• Input Sensitivity:
• Signal to Noise Ratio: 78dB (mm), 98dB (line)
• Line Output:
• Dimensions: 480 x 134 x 260 mm
• Weight: 5.5 kg
• Year: 1989

Apparently, if you wanted to use this unit with an MM cartridge, you could do so via a selector on the face plate that selects for various levels of attenuation of its phono gain or perhaps of its linestage gain.  Kind of strange. Hence the inclusion of another selector that allows the user to add phono input capacitance.  So if you really really must have a SUT in the signal path with an MC cartridge, you would need the 47K ohm load resistors, not 100 ohms, and you could select for attenuation of phono gain on the front panel. That's a minefield for the novice.

Again:

getting to the bottom of the hum thing might be a good idea too. SP-14s are older ARC preamps if memory serves (and a quick search says it was introduced in 1989...) - when was the last time the filter capacitors in it were replaced?

If the answer is 'never' then it would be a good idea to get it serviced out. Its old enough that filter caps (including those in the DC filament supply) can be failing. Failing filter caps have a way of eating power transformers in older gear and you really don't want that happening! It smells terrible and you can expect a transformer like that to be really expensive if you can even find it.

@lewm From specs listed in the SP-14 Sterephile review:

Phono stage gain: 66dB (to main out), 46dB (to tape-out). Phono sensitivity: 0.25mV at 1kHz for 0.5V output. Line stage gain: 20dB (to main out), 0dB (to tape out). Line sensitivity: 50mV RMS at 1kHz for 0.5V output.

I think the clue is that they list phono stage gain: "66dB (to main out)" BUT only "46 dB (to tape-out)". To me, that indicates they’re adding the line stage’s hearty +20dB gain to get that 66dB figure. If the phono stage were doing 66dB before its line stage, then that should have applied equally to the tape-out.

It seems like it was a thing for C-J and ARC full-function preamps of this era: standard MM phono stage, but with GOBS of downstream line-stage gain to cover users who wanted to run MC cartridges anyways. I think I recall seeing spec’d phono stage gain on some of these preamps worded like "including line stage gain". It was a weird era. Fortunately this practice has fallen out of favor, as it’s suboptimal. The extra gain for MC carts should be before the RIAA stage, where it belongs - and not in the line stage, where it just increases noise floor for modern digital sources that are 2V - 4V or more.

And I agree with @atmasphere , the hum in direct connection is concerning, and should be looked into! Also the capacitor age, though op did mention it was recently serviced...