Cartridge/arm matching

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.

I agree...the heavyweight counterweight makes a real difference. There is also a gizmo out there for adjusting the VTA. Anyway.....what don't you like about the way your setup sounds as it is?

You should get a test record. I use the Shure Era 4 obstacle
course(about 20 bucks on Ebay). It has a test for tone arm
resonance to test your arm and cartridge compatibility. You can
also get a Technics 3 gram headshell weight,if you need to add
mass to the arm (about 5 bucks on Ebay). These are very
inexpensive tools you should have if you are into vinyl.

The Dynavector 10X5 would be a great fit on the RB250. You wont miss the shelter. IMHO.

Rega RB 250 effective mass is 12g
and you know the compliance of your cartridge @ 100Hz (Japan standard)
make sure to multiply it on 1,5 or 2 to find out actual compliance at 10Hz

and then simpli check the diagram illustrates the relationship between cartridge compliance, tonearm mass and the resulting resonance frequency:

http://ortofon.com/hifi/support/cartridge-tonearm-resonance-frequency

you will see which cartridge (compliance @10Hz) will be the best match for your tonearm (effective mass) to stay in desired resonance range from 8 to 15 hz