Preamp. The preamp is the heart and central component. That is where to start. A good tubed preamp will be very musical and reliable, and give you a taste for what is to come if you continue down the path.
BTW, for the first twenty years in audio all of my equipment was solid state and then over the following thirty years one component after another got switched to tube equipment (my system is shown under my ID). My only regret was no doing it much faster.