Ok I think I'll step in and talk about vocals here. I agree with both Garnet, GacktsVanilla, and Phantom in different ways.
Klaha, Juka, and Seth all sing in a pseudo-operatic manner. You couldn't really call it straight opera because well...it's not. They keep their timbres natural for the most part, and many aspects of their styles are considerably modern. This style suits Mana's work perfectly though, as a combination of the antique and modern: baroque and rock, yes? For this reason, I don't think Mana will move away from this vocal style.
I do think Mana finds variety within this preferred vocal style. Klaha has a very even vibrato and accurate pitch placement. as well as a lot of lung power. Like it was mentioned before, he has a dark and rich vocal timbre. Juka has more of a round, romantic timbre, and a rather more boyish voice. He also has little experience and a generally untrained voice, and can't really produce natural vibrato (and really doesn't have much range at all, though it's expanded since he was playing with Hizaki). Seth has a lot of experience singing, and has a very mature and worked voice. He can produce very nice natural vibrato and has quite a bit of range too. He tends to use a very light and airy technique.
I think you get even more variety when you consider Tetsu and Gackt, too. Though they all are around a baritenor range, they have varying vocal capability.
I really like what Seth has been doing though; I think his voice is pretty much some of the best stuff I've heard these days in visual kei or goth or whatever you want to call it, but if they had to go in a new direction I'd like more of an opera-leaning man again like Klaha. It'd be interesting with the instrumental sounds of Moi dix Mois.
I don't even know how this discussion relates to the original topic...maybe I shouldn't have encouraged it!
Looks like even the wild man of M10M has his boring side ::kisaki::
