I notice improved clarity and detail on my A900 with my PPA, but truth be told, I mostly bought my PPA and source thinking I would upgrade my cans down the road. I would think an amp for the A900 isn't really necessary, they are just that efficient. By the time you are buying amps that start to improve the A900 (i.e. clarity, details) you have an amp capable of driving some monsters. For example, I've been told by one head-fier that anything below a Pimeta on an A900 is just basically a ritzy volume control.
Of course, I'm no expert--perhaps someone with more experience will have a different point of view.