Millett need fairly high voltages? What well-known "Millett" amp runs on anything other than 24V (except the Starving Student)? No offense, but there's only one family of low-voltage headphone amp designs that runs the tubes in their manufactured voltage range - that's the "Millett Hybrid" family of amps (Starving Student not included). Only space-charge tubes are designed for 12-24V on the plates as recommended in their data sheets.
The 6GM8 is a low-voltage space charge tube exception to the Millett Hybrid family, but that tube is few and far between and purchasing them is about like purchasing a collector's item. You can build an EHHA with a couple of them, or play around with modding a Melos amp that uses them, but that's about it.
You might want to check Jeff Duntemann's web page on space-charge tubes:
Those are all pretty much the truly-low-voltage tubes in existence. The Millett Hybrid family of amps use the 12AE6, 12FM6, and 12FK6 tubes.
P.S. I should add that the guys above have mentioned some very good amps, but I got a little amped up when you were asking about low-voltage tube amps and mentioned that Millett was not one. The original Millett Hybrid by Pete Millett that was published in Audio Express magazine was a trailblazer for DIY headphone amps because it was all based on 24V:
It's since been modified some (like in someone's avatar ).
Edited by tomb - 7/18/13 at 5:33pm