For using a DL103R or Shelter on your OL modded RB250, major improvements can be had using the HiFi mod. You can read about it on the analog page "Strange Tonearm Tweak" thread. It describes exactly how to make it DIY. If you don't have the time, I can make one for you. There are about 40 Audiogon users who have this mod, and can attest to its effectiveness over short and long term. It is primarily designed for low or mid compliance cartridges, and not for Grado or high-compliance cartridges.
I had a DL103R on my OL Silver tonearm when I invented this HiFi mod. It was a very satisfying improvement overall. Cost is very low. If you DIY, it is only a few dollars.
Additionally, if you add Doug Deacon's "on the fly VTF mod" it will simplify small VTF changes needed for the very VTF sensitive Shelter cartridges. Also good for the DL103R.
That helps out for your RB250, but the Graham question is still up in the air. For that, I'd still recommend trying out a ZYX. In very revealing systems, tonearm matching becomes even more critical, and perhaps the Graham is not in the ideal category for that Shelter cartridge, although many seem to be happy with the combo. In my opinion over the years, I feel that low compliance cartridges always stress any unipivot design to the max, due to large amounts of energy being fed into the arm, and sometimes this reveals some less-than-perfect sonic results. The Graham is one of the best stabilized unipivot arms, and even it seems to show some occasional shortcomings when mated to these low compliance cartridges. I have been taken to task for this opinion about unipivots many times by unipivot users, but the fact is that this kind of problem keeps popping up from users with problems resulting from these matchups. My gut feeling is that even the best unipivots are "on the edge of stability" with low compliance cartridges.
My 2 cents.
IMHO, YMMV.
