Heh, I almost picked up that Amon Amarth last night.
Only listened to a couple tracks from the Amorphis disc. If you're familiar with their old stuff, well, this is a complete 180. Vocals are clean, songs are catchy. Doesn't mean it isn't good though.
I'd recommend Elegy or Tuonela over Am Universum. Elegy, I think is the better of the two, but Tuonela is good too, just more use of clean vocals. IMO, both are way better than Am Universum.