Great explanation at least to my not always so technical mind!
The denon 103 needs much more then 100ohms. the spec sheet say more then 100ohms not 100ohms. Note also the spec's from Denon were taken at 1000ohms FYI.
I also have a Denon 103 on my second arm and I find 470 ohms to 1k ohm to be ideal. Mind you the 220 ohm setting is not bad either but the 100 ohm is not ideal at all IMO. The denon 103 really need closer to 1kohm IMO. I usually mine run it at 470ohms.
Glenn, Your personal experience is telling, but the OP seems to be saying that 500 ohms is too "harsh" sounding, and 100 ohms is "boring". Since 500 ohms is not significantly different from your idea of the optimal 470 ohms, it's possible that the harshness he experiences is due to some other element of his system. On the other hand, I have read elsewhere that some prefer to really load down the DL103, using load Rs that are near equal to the internal resistance. I don't know why this would be good, but it suggests that something about the hf response from the DL103 is best attenuated.