I tried this Germanium version of the fethead after owning the regular one since decades.
I was curious about the difference, and we tested both in turns on a very nice vocal mic with a female singer.
The difference is very very noticeable.
In my opinion, the original fethead is really perfect for a very true to microphone gain boost. I use it mostly for acoustic guitar micing, singing of course, and sometimes for cab micing on electric guitar, if the engineer can't seem to get enough room feel in there, as u can get further away from the speaker and still get some punch from it.
Now back to the germanium version, this one really shines for vocals in my opinion, as it colours and brightens the sound, a lot ! And that's mostly not what we're looking for for micing cabs and speakers, as it may get way too trebly.
That said, for voices, it seems to add a lot of definition to the sound and definitely a lot of treble, which could be too much for some, but you can always apply hpf to it during the mix. Which leaves you with a lot of singing dynamics, punch and better mic room control.
Both versions do exactly what they are meant for, A Supertool in my opinion !!