Why is it that?

JC Gold phono board in the AI is most likely the same daughter board they've been using for years. To find out, do a visual comparison or ask John Curl. I wouldn't trust any other opinion especially the manufacturer.

IMO SUTs are band-aids that ad more connections to an already delicate signal. In this case…

["ZU/DL-103 PHONO PREAMP MATCHING

The nominal output voltage of the Zu/DL-103 Mk.II is 0.3mV and the nominal internal impedance per channel is about 45 ohms. We recommend running a step-up transformer for moving magnet phono preamps. A step-up transformer is not required for phono preamps that have a moving coil (MC) setting, setting your impedance between 80 – 200 ohms. Even so, you should consider a Bob’s Devices 1131 or similar."]

So I'll go back to your original question: what is the difference between the 40 and 3 ohm settings?

This can get confusing because some SUT manufacturers list the impedance presented by the SUT (assuming its feeding a typical 47k load into the MM section of your pre) and others, like your Denon, use the figure corresponding to the internal impedance of the cart you're using. The DL103 has an internal impedance of 40 ohms, but, and this is the important part, the impedance it wants to "see" is several multiples of this (Art Dudley recommends 2-6x, Bob's Devices recommends 10x, YMMV). So if you buy these guidelines your SUT + MM stage should present somewhere between 80-400 ohms to the DL103 to sound its best.

Now, according to Denon your SUT has turns ratios that can be switched between 1:10 and 1:36. These correspond to impedance ratios of 100 and 1300 respectively (the squares of the turns ratios). To calculate the impedance presented to the cartridge you divide the input impedance of the MM stage (typically, but not always 47k ohm) by the impedance ratio of the SUT. In your case these equal 470 and 36 ohms respectively.

What I notice is that neither of these is really dead center "ideal" for a 40 ohm cartridge, though the higher setting *should* sound better. But supposing that the input impedance of your MM section is not 47k but 100k ohms (some are!), that would mean the the impedance presented to the DL103 is either 1000 or 77 ohms. In which case the 40 ohm setting might be too high, and the lower 3 ohm setting may be preferable.

Clear? as mud, I bet. :^)

And this is where I say "trust your ears"....

What CJ says is spot on, which is why I was wondering at the sonic results. The OP writes that the sound is more "romantic and richer" and moreover that there is an apparent loss of gain, at the 40-ohm setting vs the 3-ohm setting. That does not make sense, if the 40-ohm setting corresponds to an input Z of about 470R or 1000R (seen by the cartridge). That sonic description more fits what I would expect from the 3-ohm setting. Could it be that the Denon SUT is mis-wired such that the 40 setting is actually the 3 setting and vice-versa? ("Romantic", "rich", and apparent reduced cartridge output are associated with using a phono input impedance that is lower than optimal for the cartridge internal impedance. 470R input Z is not going to sound that way with a 33-ohm cartridge internal resistance.)

And FWIW, lots of DL103 users do seem to prefer oddly low phono input Z (breaking the 2X to 6X "rule"), nearly equal to the cartridge internal resistance, for best sound, even though there is some loss of gain associated with such an impedance mismatch.

"When did AI come out with the 4A? I see no mention of it on their site." Here's my second moderated response. Another reason I'm spending more time on other forums. Contact AI for the build specs and details.