I agree with jmcgrogan2, I would go with a tube line stage but if could add another tube it would be the phono and then the amps which I find to be the easiest to replace with solid state.
Phono/preamp/amp chain. One of them tube the others SS. Which one?
Perhaps any one would do if done right. What would be your preference, generally speaking? Let's assume MC cartridge. This move could be permanent or could be the first step on the way to full tube system. I would probably choose the phono.