You, my friend, have touched upon one of my favorite topics. I can go on for days about this. I'll cover Davey and Jade right now and try and come back to Hunter and Adam in a bit.
Alright, let's talk Davey. I'm a writer by trade/profession, and I have him to thank as one of my biggest influences. When I first listened to them as a kid, I was struck by the sophistication of his lyrics compared to those of other "punk" lyricists. There was something almost Baudelarian about his work. That really spoke to me, a kid who grew up on Poe and Lovecraft and Dickinson.
His vocal progression is stunning. There's a level of confidence that he has gained over the years that has allowed him to take more risks vocally and career wise that I think we can all appreciate. I really think that, despite how polarizing the album is, Decemberunderground was a breakthrough for all of them, and for Davey it was an era in which he could try new things vocally that have only been perfected in the subsequent years.
Of course, I admire everything he's done on the side, but that's a whole new argument.
And on to Jade:
He's admitted that the bands he was in before AFI were always a bit more melodic than was pre-Jade AFI, and I think that is a good thing. You could tell he was trying out incorporating melodies into AFI's sound with Black Sails, and he just became braver and braver. Davey and Jade have mentioned before how much they admire each other's work and how confident they are with each other's boundaries; I think that that knowledge and intimacy has allowed for them to push each other. For Davey, Jade knows his limits vocally. And for Jade, Davey has a tendency to always speak highly of him, and I think that that really lends Jade confidence to try new sounds and push their style in different directions. I also think that because they work on different projects, that Jade knows that should something not work for AFI, he can try it somewhere else (he did say how Love Like Winter was going to be a Blaqk Audio track). Jade's level of confidence in his abilities has grown through the years, and we can definitely hear that in the music.
I also think, for both of them, that as they get older and move away from boundaries of "that's punk" and "that's not punk," that they are able to just create and not worry about genre or credibility anymore (not that I think that was ever a conscious concern; I just think that as teenagers and young twenty-somethings, you're going to want to fit into your chosen crowd, and subconsciously that's going to lead you to making decisions that keep you within the "rules" of that crowd).
The guitar work has become more complex, though Jade does find a great balance between punk riffs and more technical guitar work (something he talks about in recent interviews); the vocals have been pushed, and the lyrics are becoming more and more elegant.
They are a song-writing duo that continues to evolve and grow elegantly.