At many audio shows, the Zu Audio room features their models playing hard rock and metal music playing at extremely high volume. The speakers are quite capable at delivering at high volume levels.
Full range single driver systems cannot deliver that kind of volume level (the Nenuphars delivered plenty volume for my particular taste) and actually excel at delivering a vibrant sound at quite low volume levels. Although they are not particularly efficient, they are probably efficient enough for use with some SET amplifiers if one does not demand extreme volume output or use in a very large room.
Use with a single-ended 300B amp? I don't know, I never heard them used with that kind of amplifier. But, I guess they are not well suited for smaller single-ended amps, like those running 45s, given my experience with a 99 db/w 8ohm speaker (I've run this speaker with pushpull 45s, parallel SET 2a3s and pushpull 349s; single ended 45s were okay, but marginal with certain kinds of music).