The idea of using tubes for a zero gain output buffer is contrary to my understanding of what tubes are really good at: tubes are good for voltage gain, they are not so good at driving current (same with JFETs). A unity gain buffer circuit is used to drive current; my experience suggests that MOSFETs would be the way to go, or (hate to say it) you might want to experiment with some of the diamond buffer IC chips as well. As mentioned before, to make a really good buffer stage you are going to need superb power supply rails, virtually zero noise, and very low output impedance, if you want the buffer to perform at a really high level.
Tubes and JFETs have plenty good enough current delivery for purposes of a preamp buffer. Moreover the job of preamps isn't so much to deliver current as to deliver the voltage signal (the music). With amps having input impedances of 10k or more even a full volume (unattenuated) line stage signal of say 2 V would only have to deliver 2/10,000 = 0.2 ma of current. It's ain't nothing, but it ain't much either.
Tubes can and are used quite often as buffers when configured as unity gain cathode followers as opposed to their more common configuration as voltage amplifiers on their plate side. John Broskie (widely recognized as a tube design expert) has quite a lot to say about cathode followers. Here's one of his articles on the subject:
http://www.tubecad.com/2011/08/blog0212.htmAnd then there's Nelson Pass with his JFET B1 Buffer where much like tube buffers the JFETs are used as followers in a unity gain buffer. This is a very effective preamp buffer design and has been widely adopted within the DIY community.
https://www.passdiy.com/project/preamplifiers/b1-buffer-preampI do agree that in the end it's all about the power supply. That's what all audio preamps (active) and amps are all about really; their outputs are modulated power supply signals. Garbage in, garbage out. That is perhaps the most compelling reason to avoid introducing a buffer into a passive preamp which has proven it doesn't need one to deliver awesome performance.