So where should the tubes be?

Tube "sound" is all about the design and engineering of the electronics.  I have heard very neutral tube equipment, not warm, lush or thick.  Don't have that "tube sound" or "solid state sound."  Just music.  

Tubes belong wherever they sound right to the owner.  

I like either an all tube front end which is at least amp and pre amp for a tube system, but it will all be up to money and maintenance cost, the more tubes in the chain , the more up keep on the entire system. The one thing i can say is that most tube phono stages and tube tuners require a lot more care to keep quiet and aligned than there solid state counterparts. A tube phono stage can also be a little noisy on high efficiency speakers, especially when run in a balanced configuration.

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.