So where should the tubes be?

Wherever you want. No rules. It’s up to you. Impedance matching between components where tubes are involved is strongly advised however. If you go about it randomly it may sound ok but not best. 

I've been running tubes in my system for over 35 years now. Amp, preamp, phono stage, headphone amp and CD player. I always had tube friendly speakers and the music has been glorious- to my ears. But it's time for a change. Just this week I switched to a Luxman all in one integrated and CD player. I know it's going to take some time getting use to SS, but so far I'm impressed. Now I have a boatload of gear to unload, 2 boxes full of IC's and PC's as well as a tube collection. 

All tubes or just one tube, it's all good if matched properly. For very high power systems tubes get a bit pricey and generate a lot of heat. I use solid state for high powered systems. 

Class A ss amp. Tupe pre amp and phono amp. You could also add tube dac. That’s what works for me. 

Isolid state has evolved a lot ,fet, mosfets have several characteristics 
like tubes no more bright sounding ,thst being said a tube preamp is my favorite 
spot , too much tubes a bit too soft on leading edge . I prefer SS the dynamics control and current delivery have their advantages especially to control 
In Bass  drivers . I have used tubes for 40 years , I no longer feel tubes are necessary for detailed Rich , natural sounding music , but in the preamp section 
I feel the best position with a Solid State power amp .
my New Coda SCIB integrated with VH Audio Odam capacitors in the class A 
preamp section ,it sounds very natural sounding ,with the Bricasti dac-Streamer front end a very balanced sounding system.Everyone has their own  preferences .