Cartridge/arm matching

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

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?