I've played Vandoren V16s (T75 and T95), the metal ones, Lawtons (8*B and BB), metal again, and a rubber Berg. The V16s work pretty well, especially the T75 but it's a tad bright and I gave up because of the problems discussed a while ago with them,i.e. It's very difficult to find and maintain a core and doesn't survive well in a nig or noisy environment. Small rooms with a 4 or a 5 piece and they're great. The Lawtons are powerful beasts and work very well but they tend to dominate the horn. I've also tried a metal Link but that was thin and bright.
My main piece now is an oldish, rubber Berg 110/1/M. This plays beautifully across the horn, rich and dark at the bottom, resonant in the middle and still strong and full at the top. Edgy or dark as you choose. I recall reading that these were the preferred pieces for S20s, that would probably be before the designer pieces we have now. I was sceptical for a long time, not least because of the difficulty in finding a decent Berg which is substantial. I've done a lot of work on this one getting it into shape. I let a 110/2/M go a couple of years ago and always regretted it but at least I knew that a good one was possible. The new ones are different with a higher baffle I think, but ones from the 70s or 80s are more complex, it seems to me.
I've assumed that an RPC would be a great fit for a S20 and had thought of searching one out. It would be interesting to hear your experiences with them Grumps.