Ambre Extreme differs from L'Eau d'Ambre in more than just concentration. It's sweeter, hits the vanillic elements more, and has more mid and basenotes, including spice. I'm not saying it's better (or worse), just different.
Per Luckyscent (L'Artisan's site is much more coy about specifics) the notes are: "Cinnamon, pepper, cardamom, nutmeg, amber, vanilla, rose, patchouli, sandalwood, benzoin, tonka bean." In L'Eau d'Ambre, the rose is almost palpable, as well as geranium (if not geranium rather than rose - some perfumers make the substitution, claiming "rose geranium," without distinction), and oppoponax. The patch is a nice, soft one, and the sandalwood is powdery.
Ambre Extreme ramps up the spices, especially the nutmeg, and the vanilla. It's more gourmand, and yet there's still that L'Artisan-y feel, almost floral, of lightness, of waft. It comes through in a somewhat powder feel, though not powder as many old school ambers are powder.
How I'd describe this is as a gourmand, spicy amber with real last. It's a next morning/2 hours later soft waft of rich, "perfume-y-ness," that soap/skin/sandalwood feel of good french perfume. I love this, and find myself reaching for it often, especially when I want a slightly gourmand amber, but not the whole cinnamon bun with vanilla overkill feel, a thing which can have it's place.
I'd put this as a sister of Molinard, and far from the gournand clouds of Nez a Nez. It also has a little feel of the beautiful Kuumba Made ambers, though far less sweet and dense, and much drier and more spicy/floral. I know this sounds goofy, but there really is a L'Artisan unity to this fragrance, as much as it may seem out of their expected range.