teflon tape and rega RB300 arm

If you don't mind taking the arm apart, use 3:1 heat shrinking tube around the arm tube and heat it up to fit snuggly.

I use the Teflon tape tweak on my Linn Basic arm and the result is very beneficial; a noticeable reduction of resonance. BUT, the Linn is a low mass arm and I don't think the rb300 is, so results may vary.

The only drawback for me is the tonearm now is kind of ugly, but makes for a good topic of conversation.

Why not use premium grade vinyl electrical tape? I have also heard about people using liquid paper to damp tone arms.

I could have used pure silicon tape, or 3m 33 electrical tape but did not want to add extra weight, the teflon is much lower in weight than other tapes.

It's the weight that provides the damping. Not sure if teflon tape has the right damping properties. The good thing, it cost almost nothing to give it a try.