I've heard Witchbreed before and very much like the vocals, though not sure about the other band.
Problem with a lot of these female fronted metal bands is that the vocals seem too recessed (if that makes any sense), and almost always overpowered by the instrumentals, making it nearly impossible to listen to the 'timbre' and texture of the vocals; the finer details of the voice are completely lost. When I said I'm interested primarily in the technical aspect of the vocals, I was being completely serious. This is why I like Norah Jones, because her music seems to be mixed in such a way that you can hear huge amounts of detail in her voice, especially with a decent pair of headphones that can do midrange properly - may sound odd to some people, but I find it incredibly relaxing. Other female artists that have such attributes in their music are Alison Krauss, Sarah Mclachlan, Tori Amos, Beth Gibbons, Paula Cole, Heather Nova, Cat Power, etc. I don't give a toss about the instrumentals and lyrics, but the vocals sound amazing. That said, if the instrumentals are also good, then all the better (e.g. The Gathering).
Put simply, I want music with
talented female singers, where the vocals are upfront and the main focus of the music.
Finally, since I'm replying to you Lysander, I quite like the vocals of the singer of Vas (which I found from your last.fm page). Very beautiful and amazingly well mixed (vocals have incredible detail). Fact that I enjoyed it despite not understanding a single word, speaks volumes.