This is interesting, as just this evening I put my HHKB aside to try out the Novatouch and RF87U 55g again. I was struck by the harsh bottoming-out sound of the Novatouch, in addition to the return-stroke clack and stabilizer rattle. In my case, I thought the bottoming-out sound of the NT might have been accentuated by the Leopold PBT keycaps.
I've developed silencing rings for Topre-switch keyboards, but ironically, on my own keyboards, I have thus far installed them only on my HHKBs -- one modded with 55g domes, the other with stock 45g domes. Others have reported a dramatic difference in the sound of the NT by installing silencing rings, but they should only affect the return-stroke sound and not the bottoming-out sound.
Because the NT has Cherry mx-compatible stems, you can also install conventional O-rings on the keycap stems to damp the bottoming out sound. I recall trying O-rings on the NT when I first received it, but I didn't like the effect on the feel of the keyboard.
Overall, although I like the fact that I could put Cherry mx keycaps on the NT, I prefer the sound and feel of the RF, but I should try installing silencing rings on both boards before reaching a final verdict.