Tube or R-2R DAC for Tube System

generalizations are not true and it really comes down to implementation.

Bingo. When they are true its someone using a brute forcet echnology to overcome design limitations.  "I cant make it warm and natural so I'll throw in some tubes".  Pass, with apologies to Nelson LOL.

Don’t put yourself in a corner. There are many excellent dacs 
besides R2R dacs  the classic Multibit dacs are used still by many companies like Analog devises 1955 chips used by Briscati  BB1704 used by several companies ,and other Multibit dacs 
which are R2R in practice they are all incorporated on one chipset 
and were very precision built all laser trimmed and very time consuming that’s a big reason they were discontinued .
in fact Briscati upper dacs incorporate a choice of filters ,Multibit dac ,as well as R2R dac, as well as a great Roon ready streamer module.seperate DSD ,as well as high quality analog preamp section .

In my own analysis of this it seems like the most compelling DACs in these categories may be:
R2R Ladder:
Denafrips Terminator Plus
Holo Audio May KTE
Tube:
Doge Audio Clarity 7

Others may have other suggestions. 

We make both tube DACs.  One with the Sabre 32 bit chip and the other with the R2R board.  The Sabre uses a single 6SN7 with a single rectifier (GZ34 - 5R4, etc.) and the R2R is a 101D direct heated triode design.  They have similar sound characteristics, but the 101D is a much more sophisticated design.

Happy Listening. 

Solid-state IC DACs verboten? I feed the output from one into an integrated tube amplifier. Maybe that is how the system produces a well-defined and musical sound.