Tube or R-2R DAC for Tube System

Learning now!  Entirely passive seems like a tough proposition since the output will have a variable impedance and a high one at that.  For a home/hobby project where you can control the next stage maybe its feasible - but in commercial proactive that sounds, to me, like suicide.

Transformers are a natural for the task, but are typically big and expensive ( and most that are not , are not very good) . I have not experimented with any custom ones.  I plan to use discrete circuitry, combining FETs and BJTs where each as an advantage - and keeping it pretty simple. (what was it that Einstein said? "as simple as possible, but no simpler" :-)

The output characteristics of bitstream DACs vary quite a bit too- some only give a voltage output - meaning an onboard opamp i presume. The real world gets messy.   But again, still learning. All my DACish work so far has been on the other end of the chain - input, isolation, clocking etc. 

@itsjustme, 
 Some DAC designers prefer the sound and simplicity of a passive resistor and want to avoid the use of OP-amps due to what they consider to introduce sonic degrading NFB. I do recognize that another approach is active I/V conversion via a discrete circuit again avoiding OP-amps.

 I know that Audio Note utilizes transformers for I/V conversion duty. Given their reputation for very good sounding DACs I have to assume that the chosen transformers are of excellent quality. 
Charles 

@charles1dadsI
I must be missing your point,since i addressed why one of those options is commonplace and sub optimal, and the other has serious issues.  Could you clarify what you are pointing out, or want to know?
G

Actually just citing some successful  alternative examples that  DAC designers apply to avoid reliance on the ubiquitous OP-amp. My DAC I/V conversion is via a single passive resistor and has very  good sound quality in my opinion.  SW1X audio for example offers the choice of active transistor discrete or passive resistor with good outcomes from either option. I/V conversion has an influential role in determining sound quality that I believe is underappreciated by many.
Charles 

As others have said, it's not a tube vs r2r comparison, but rather tube vs. solid state and/or r2r vs delta-sigma. 

My main amp is a Line Magnetic LM-218ia -- an 845-based integrated SET amp. I have two DACs: a Black Ice D-S Tube DAC (GlassFX Tube DAC DAC DSD Wifi) and an Audio GD r2r Solid State DAC (R2R-R1). They are both connected to the SET amp and both play through the same Omega Outlaw Super 3XRS speakers (with built in 8" subs).

*the Black Ice was modified by Radu Tarta with some nice upgrades.

I find both of these DACs to be excellent, and honestly they are more similar than they are different even though they are using different conversion techniques and utilizing different output stages. In neither case would I say one or the other was a bad match for my SET amp -- they are both excellent.

If pressed I would say that I prefer the Audio-GD. I think it's a bit more revealing and I find the imaging and separation between instruments to be slightly more defined. Both of these DACs are in the $900-$1200 range and I just think they are both a testament to the quality of sound that is attainable at a relatively accessible price (by audiophile standards where an interconnect can cost as much and does far less).

-Chris