I'm not sure that every Aman is special enough to justify the prices. However, some undoubtedly are worth the extra and of the Amans I have been to these are - Amantaka, Amansara and Amanpulo. I think that Amanpuri and all the Bali places are equalled if not surpassed by other hotels in those locations; the competition is simply too great. Obviously Amanjiwo is out on its own in that location, but I've not been there.
I must say that the Amans in Morocco, Montenegro, Venice, Greece, Turkey and Vietnam don't interest me at all.