Okay, Twylie, I've been down this road, and helped alot of others down it too.
First, your RB900 is a very good arm, I wouldn't knock it.
Second, the modded RB250 will outperform it at half the price.
Third, the OL Silver tonearm will knock them both in the dirt.
Okay, now we know the rankings, so what do we do?
If you want a OL modded RB250, which is called the OL-1, you should buy it already done. It will cost the same if you do it yourself, and you can be sure it's done right if you buy it from OL. The end stub has a special torque setting, and must be done right. Also, it is possible to injure the bearings if you don't know how to do it. Just get the OL-1 arm already done if that arm is what you want.
No sense in having your RB900 modded because you can get the OL-1 for half your arm's cost, and the OL-1 will outperform a fully modded RB900, due to the presence of the spring loaded VTF adjuster on the 900, that is not present on the RB250. That's the difference.
Buying the OL Silver tonearm is the best thing you could do, because it outperforms nearly anything, and costs a little less than your RB900 does. It is a little more money than the other options, though.
That being said, there are some things that will improve the sound of any of these arms, regardless of their state of modification or price. These are the Expressimo Audio HeavyWeight counterweight, and VTA adjuster. The OL Silver already has a VTA adjuster included. The Heavyweight is about $90, and is worth every penny, for all of these arms, including the OL Silver. VTA adjustment is mandatory, so you have to get one of those.
Now I will shamelessly plug my own product. On the A'gon classified, under Analog, you will see an ad for the HiFi Rega tonearm upgrade. You can read all about it on there.And see the photos of it on my OL Silver tonearm w/Heavyweight. I invented it right here on the Audiogon forum, and had Audiogon members do the testing. All were ecstatic about the improvement levels, and there were no negative comments at all. It will put any of these tonearms into the top league, sonically. This thing works, and it works well. It may look simple and inexpensive, but it will do more to elevate the performance of your arm, than any other single mod you can make. It is worth every penny of the cost, for the performance increases, even if it is only a set of simple weights. After all, the Heavyweight is $90, and it is only a simple weight also.
If you get an OL Silver, a Heavyweight, and the HiFi mod, you will have an arm that is in the State-of-the-Art category for under $1k. That is what I did, and is what I own, and I can categorically state that this is the best tonearm I have ever heard, bar none.
Also, get rid of the Sonata, and get a Shelter 501. If that is too much money, get a Denon DL103R from Martin on the Audiogon classifieds for $230. I use this cartridge myself, and it is a mind-blower on the modded OL Silver. It has .27mv output, so you need to have about 60-65db of gain in your MC phono stage(or a step-up transformer). You can probably sell your used Sonata for more than the DL103R will cost new. The DL103R will stomp the Sonata into the dirt. And it won't have susceptibility to hum like the Grados do. It will work on all the arms we discussed here.
This is the ticket for the "best bang for the buck" arm/cart setup in all of audio. If you work it right, and sell your other arm/cart, you can probably get into this setup for less than $250, out of pocket.
