I would speculate that ambre essence is probably a straight amber. Have you ever compared an amber resin to an amber FO? Resin is luxurious and a sniffgasm.
I did have some show up 2 1/2 weeks into the cure, but it was a horribly humid summer and the ac died and one of the oils I was trying out at the time was not a good cosmetic grade either, but I agree with you 100%.
Kept in a cool dark place should help them. Keep in mind though that they can go south(bad) in less than a year. I'm dealing with a slew of them that aren't a year old yet that have lost their whiff and while it's been a week of disappointments, at least I know not to reorder them. On the flip side I have another bottle that's 15 years old or so and it's just fine. Too bad most of them aren't like that one bottle.