Cartridge/arm matching

Actually, yes. ;-)

The compliance Denon quotes for the DL-103 is based on a measurement at a 100Hz, which is not industry standard (or particularly useful). When measured at the industry standard (and more useful) 10Hz, the DL103's compliance works out to be approximately 11x10-6cm/dyne, which is actually a bit higher than the Shelter's.

Fully agree with the rest of your post.

The 103R is similar in compliance to the Shelter, so it's unlikely to be a better match in this regard.

I wouldn't go buying other cartridges trying to fix a problem you haven't even heard. Is there some aspect of the sonics you dislike? Or are you just afflicted with audiophilia nervosa? (A common malady in these parts!)

If there is a sonic issue, and if insufficient system mass for the cartridge compliance is the cause, a simple fix would be to add mass to the headshell. You could trial this by taping a nickel (= 5gm) to the headshell. Rebalance the arm and set VTF to the same value you use now. Listen and compare. If you like the improvement, there are various ways to make the change more secure and better looking.

The formula noted by Chakster is correct but it has a very limited purpose, viz., assuring that a particular cartridge/arm combination resonates somewhere around 8-15Hz. A resonance frequency in this range is presumed to avoid excitation by either musical signals cut in the grooves or typical record warps.

The Shelter/Rega combo falls inside this safe range, as do probably 90% of all cartridge/arm combinations. No need to fuss any further about the math, it won't tell you anything more and certainly won't predict the sonic behavior of any cartridge/tonearm combination. For that there is only one test, the ears of the listener.

As I originally posted and Stringreen repeated, what sonic issues (if any) are you trying to address?