Added an SUT...not sure I understood this

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...

In a 30 year old unit, hum is not surprising and it need not have anything to do with the subject. Your logic makes some sense regarding phono stage gain. Good point about phono gain at tape outputs being 46db. The fact remains that a SUT is not needed.

Seems to me, you have DC in the signal. DC will not pass through the transformer(s), which is why the hum is going away