Added an SUT...not sure I understood this

@noromance the SP14 does not have an MC setting.  The expectation was that one would either just crank up the gain or use SUT or headamp.

@jasonbourne52 I wish I had someone else in my orbit with an SUT to borrow, but my audio clan is very anti-vinyl, so no joy there.  It's good to know my expectations were not crazy.

The AU-320 has a built-in cable for output, and maybe I can find someone who can solder in a set of jacks instead, since the cable is pretty generic.

@dweller the SP14 has a built-in phono stage.

Well, as I said, I removed the load resistor when I installed the SUT.  Thus, the SUT is driving my preamp's 47k ohm input impedance without modification.  The pre-amp was just serviced and spec'd out by ARC, so I know there's no chance its impedance or gain are other than as designed.

It sounds like everyone agrees with me on what adding the SUT should result in, both in terms of gain and tonality.  It's just that is not what is happening.  

@mulveling Thanks.  Clearly there is something wrong with this SUT.  There's no possibility of wiring it backwards since it has a built-in output cable and two sets of RCA jacks for inputs.

You are right about the gain on my line stage.  When playing from a digital source, my light fixtures start rattling before I reach the 10 o'clock position.

@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, which otherwise has been  persistent despite lots of attempts to address it, including having ARC work on the pre-amp twice.

You are correct that through the SUT I lose some resolution,  although I was blaming that on the crappy built-in output cable. I figured I could find a way to replace that but maybe I should, as you say, forget it.

@intactaudio Connecting to the 40 ohm tap results in less gain that connecting the cartridge directly to the MM input.  On the front panel of the SUT, there is a selector to bypass the transformer, to use the 40-ohm tap, or to use the 3-ohm tap, and the  gain achieved by each of these is exactly the opposite of what you would expect:

Bypass - loudest, 40-ohm tap - quieter, 3-ohm tap - quieter still.

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.

@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. 

Yes, JCarr sort of contradicts himself a bit and ends up being half right.  The reason a cartridge sounds different under a heavy load is because the low load impedance suppresses output voltage. But a preamp that is uninterested in output voltage would not be affected by this phenomenon.  In the case of the IV preamp, the low input impedance (not quite the same as load impedance) results in high current flow through the preamp.  It does for the IV preamp what a high load & input impedance does for a traditional preamp--maximizes the electrical signal that the preamp is using.

Changes in sonic character between the two approaches may be based on inherent differences in the architectures, or based on differences between the way the cartridge's--let's call it "frequency response"--as expressed by its current output varies from its "frequency response" as expressed by its voltage output, but those changes cannot be attributable to the effect of loading on voltage output since the IV preamp effectively ignores that effect.

from the cartridge POV, how can 3Ω be different than 3Ω?  I am speaking primarily about the effects of the load on the electromechanical behavior of the cartridge and what impact that may have on the sound.

You are correct in your conclusion, but IMO asking the wrong question.  Rather than looking at it from the cartridge's perspective, consider the pre-amp's perspective.  The pre-amp is the device that will be passing along a signal to the next component in the system.  Both IV and traditional preamps "see" the entire "electromechanical behavior of the cartridge" but neither pays attention to the full set of that behavior.  One type of circuit pays attention primarily to the current, and the other, voltage.  Thus the two methods can look at a cartridge behaving in very different ways but end up paying attention to--and passing along--signals that are similar in character; i.e., if the cartridge's current under heavy load is similar in character to its voltage under a light load, expect an IV preamp to pick up a similar signal from the cartridge through its low impedance input terminals as a traditional preamp would through its high impedance (and lightly loaded) input terminals.

@intactaudio I misunderstood the intention behind your post then.

Unfortunately, the question you have asked has a rather uninteresting answer. Everyone on this thread has been assuming I determined that the best sounding load for my cart was a large deviation from spec, apparently because one participant loudly and repeatedly felt the need to insist I did not know how to read the instructions that came with my cartridge. I have attempted to clear up that confusion by asserting that my cart in fact has a 20 ohm spec, but apparently I don't know whatever secret handshake a person needs to know in order to be taken at one's word on these forums.

But none of that changes the fact that I am loading my cartridge at spec, which might render your question moot. I do agree with you, though, that if I achieved better sound at a different load than that recommended, I would use what sounds best. 

@dover Feel free to dunk, it's apparently important to you.  Yes, you are right that I initially indicated the incorrect sub-species of the OC9. You remain incorrect, however, about the correct loading with the SP14, which contains neither a head-amp or an SUT built in.