Added an SUT...not sure I understood this

Well this has been interesting, and now the mystery is solved.  The prize goes to @mulveling for correctly deducing that when ARC last serviced the SP-14, they soldered 100-ohm load resistors in there.  They did know I was using an MC cartridge, but wow it would have been nice if they had said something.

Anyway, this explains a lot.  Not only does it explain why the SUT behaves as it has, it also explains why when not using the SUT, the cart sounds better when I load the tonearm with 22 ohms than when I load it at 20 ohms, which is the spec for this cartridge.

I am definitely not going anywhere near that circuit board with a soldering iron, so there will be no more experimenting with this SUT for me.   I'm going to try 25 ohms on the back of my arm, and if I like that sound, start tracing everything I can to find a way to eliminate that last remaining hum.  Thanks to everyone who contributed.

@woofhaven1992

the cart sounds better when I load the tonearm with 22 ohms than when I load it at 20 ohms, which is the spec for this cartridge.

NO

How many times do I have to post. That is not the spec.

The recommended loading for your cartridge is Min 100 ohms.

Min means minimum

It is not 20 ohms

It is not 22 ohms

It is not 40 ohms

Look at the Audio Technics website if you don’t believe me

@dover The sheet that came with my cart says 20 ohms. AT has not been consistent with their specs on the OC9 series.  More importantly, it sounds better around 20 ohms than at 100.  Plenty of people agree with me on this.  I'm not going to load it at 100 just because you won't stop yelling at me.  I'm going to load it at whatever achieves the best sound in my system. 

@woofhaven1992 

The sheet that came with my cart says 20 ohms.

I don't believe this. All OC9's are recommended 100ohm minimum. I used to sell them. If it sounds better at 20 ohms, there must be something wrong with your system.

Perhaps you could post a picture of the sheet you are referring to.

You came here for help, and several knowledgeable people have tried to help. You ought to be open minded about accepting advice, since you clearly do not have an in depth understanding of what you’re doing. There’s nothing wrong with that, unless you continue to insist on many of your misconceptions. Dover pasted the spec sheet here, so you can see that the minimum load R is 100 ohms. At 20 or 22 ohms the cartridge ought to sound weak and dull, because a large fraction of its output would be lost to ground and high frequencies would be rolled off. The fact that you found 100 ohm load resistance in your unit proves you’ve been connecting the SUT to its MC inputs, because there’s no way ARC would have installed 100 ohms at MM inputs. So if you put your SUT away, just directly drive your MC inputs and sayonara. Like Raul said.