RB300 Mod -OR- RB250 Mod, Which you feel is better

Dave, the RB300 has a tapered arm tube, which should be superior in resonance characteristics to the RB250, which has a straight tube.
With the removal of the spring, and your other mods, you should be better than the RB250, or at least just as good.
You really should get the end-stub modification,if you haven't already, as that is the most effective mod for the Rega arms.
Also, shrink a small piece of heat-shrink tubing onto the finger lift, since this is known to be microphonic on the Rega arms.
Sometimes a small piece of damping material about 1/3 of the way out from the pivot, is useful, as this is the resonant point on the Rega arms.

The rb-300's vertical bearings are attached on one side and the other side where the spring is attached only is attached by the spring itself. (I have dissasembled a rb-300 and seen this). I have not disassembled a rb-250, but it appears as though it may be attached at both sides of the bearing. Does anyone know if this is true?
I did not know that the armtubes of the rb-250 and 300 were different. TWL, you seem to have experience and/or knowledge of this resonence point 1/3 up, do you know if it is the same on both armtubes?
One last question, and I should think this is an important one- what is the difference on bearings betwenn the rb-250, 300, and 600? It seems that all I have ever read about are rumors, just as real rega's are 'supposed' to be of higher quality (or not) than OEM arms made by rega.

The 1/3 point for resonance control on Rega arms works for the 250 and I've heard that is will also be effective on the tapered tubes of the higher models too.

The RB250 has adjustable bearings on both sides of the arm tube. My OL Silver takes this one step further, and uses better bearings, AND re-locates the bearings out into the yoke instead of in the arm. Way more stable.

Rega states that the bearings are upgraded as you go up the model line. I have never disassembled all these arms to check that out. The RB250 bearings seem to be sufficient, as long as they are well adjusted.

I doubt that the OEM Rega arms are not as good as "real" Rega arms. That would require a different assembly procedure, and that is unlikely. They are most probably exactly the same, with different names printed on them. I don't think that Rega would go out of their way to make a lower quality arm, that would be associated to them anyway. That would damage their reputation, and the reputations of the TT makers buying those arms, and that would not be good for anyone.