You can probably find a bunch of interviews in which the band members answer this obvious question over and over again.
I remember back in the day, when I actually gave a shit about the bands-i-listen-to's personal lives and opinions, I read or watched a few interviews in which they answered that question.
Unfortunately, I don't really remember the answer. But obviously, when they were younger, they listened to the classic bands of the day, like Black Sabbath I believe, stuff like that... Don't quote me on that.