I had my FR-66s modded by Richard Mak with great results. My use is pretty unorthodox, though. I use the FR arm because I need to track heavy (5 grams) with 78s and the FR arm is probably one of the best-suited if not the best for such a task. Richard upgraded the bearings to ceramic and direct-wired it RCAs to pins. I’m sure the modifications hurt the resale value, but I don’t really care.
I realized later in the game that he offers the 'upgrade' from FR64S to FR66s. I probably would have just bought a FR64s and upgraded the arm tube rather than buying a used FR66s on Yahoo Auctions Japan, but live and learn.