The overwhelming opinion among Thorens experts in Germany is that the Rega 250 is not a significant improvement over the often underrated stock TP16MkII arm, especially since a Rega costs twice the used price of the Thorens.
Thorens TD 166 MkII with SME tone arm
I'm planning on upgrading my TD 166 Mk II with an SME 309, IV or V tone arm. Has any one done this? any advise? I haven't bought the tone arm yet.