So where should the tubes be?

They "should" be wherever you can access easily for maintenance.
Or not

I first added components designs using tubes in my Preamp and Phonostage, the CD player… then finally amp and DAC. The final switch to all tube equipment moved the system significantly to the musical end of the spectrum. Having all tubes just increased the realism and rhythm and pace so significantly… very synergistically. Highly recommended if well implemented.

Tubes should only be used where you want it to sound like music.