I tried to find enough technical information to make sense of this question, and discovered instead that both Denon and Sowter web sites seem have issues. I think Sowter has many obsolete web pages and not all the links have been updated yet; Denon only lists a few of their cartridges - perhaps the others are no longer in production? Anyhow, I'll dive into the technology in case anyone is interested:
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Basically, you want two things - the stepup ratio to make 3mV-5mV at the Eros input, and enough impedance to maximize the cartridge performance. That last one is the killer - transformer makers say one thing, cartridge makers say another, and the internet says all kinds of things ...
Usually the load resistance is made to be much larger than the cartridge resistance (usually more than 10 times greater) which means the cart is nearly unloaded. Loading the cartridge with a smaller resistance reduces the signal voltage and damps stylus resonance(s). Some people use that damping to optimize the load on the cartridge, but 1) the decreased output voltage is severe, and 2) the damping may compensate for other high frequency problems, such as the tweeter in the speaker being used, or the listener's hearing. So everybody's result is different, especially on the internet ... :^)
For example, 10:1 stepup presents a load of 470 ohms (into the 47K preamp input). That's more than 10 times the DL-S1's 40 ohms, but the output is still reduced to 1.35mV.
With a 20:1 stepup feeding a 47Kohm Eros input, the load presented is about 120 ohms, and the voltage losses reduce the ratio to 14 for a net output of 2.1mV to the Eros, still not ideal.
You might think that a 30:1 transformer would be the answer, but that gives a cartridge load of 52 ohms and a net stepup of 17 and 2.5mV - not much improvement at all (and probably way too much damping).
Now, some say the optimum for DL-S1 is a 100 ohm load, so you might be OK with a 20:1 ratio - Eros will be a little hissy with tube rush, though it's probably masked when music is playing. But you lose the option of trying less damping, which is often recommended on the internet.
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On the other question, the DL-103 has twice the voltage (0.3mV) at the same 40-ohm resistance, so it is better suited to a 10:1 stepup, net output 2.7mV. Still a little low though. With 20:1 you'd get 4.1mV, in the sweet spot but still with a maximum 120-ohm load on the cartridge.
DL-103R has a lower resistance of 14 ohms and nearly the same output - 0.30mV (Sowter) or 0.25mV (Denon). That would reduce the losses, but won't make a big difference.
