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"....
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"....