Like Gunbei, I have a Bel Canto SETi40, which uses 845 tubes. For some reason, it wont work with the 845B/M tubes, but works perfectly with Valve Art 845s, which cost me $65 for a pair. For the purposes you describe, they are perfect.
Female vocals and individual stringed instruments are really, really good. I cant comment on 300s.
Female vocals and individual stringed instruments are really, really good. I cant comment on 300s.