I know but does it have to be?
Not for me.
I trialed and experimented with the Fiio K11 R2R for the full 30 days to sample/hear the "warmth" that R2R is designed and engineered to produce, to enhance, etc., trying all the inputs and outputs, connecting it to my stereo, DAPs - including the Fiio M21 which I did keep, and headphones, including other new equipment I was also trialing over the holidays and beyond.
I learned that my systems, my equipment are sufficiently, satisfyingly "warm", etc., actually, understandably because that is how I have shopped and equipped my setups purposefully for ages, including many of my recordings in their various forms.
I find -again- that what I have is joyful and sufficient, and returned the very nice K11 simply and logically because it did not
add sufficient listening value. It was an instructive and pleasant learning experience and I'll continue to trial what Fiio and other impressive engineering does with my ways and means, and for/to my favorite MUSIC.
Perhaps for others' sources and tastes...