I've got them both, and I've A/B'd them before. All I remember is the 100's sound a lot better. I don't recal one being louder than the other.
Also, I'm wondering what's going on if you're able to turn up your ipod to 100% volume. I don't think I've ever had a piece of gear that I've been able to turn all the way up without it distorting at some point. I mean, I'm big on loud myself... I've got a HUGE PA, I play in a pretty damn loud band, I go to very loud shows, and I play drums without ear protection all the time.... some of these are almost 120db environments, but with my Zen Xtra at about 50-60% it's plenty loud. If I were at 100%, the players amp would be cliping, my headphones would be melting, and my vision would be blurring.
Maybe you're already deaf and you don't know it.