CassSept said:I thought thatthey are both the filter and the source of alchemy? It definitely was the filter (thus, as you said, in their final standoff when they were granted energy of planet's crust they could go into hax mode), but also as the actual source as noted by Mei before the fight with Envy in Briggs that "this country's alchemy feels like people dying" or whatever (I don't exactly remember what she said, but it was something along these lines).
No, she said it felt like there were dead people crawling underneath her ever since she entered the country. it is said at several points within the manga and brotherhood that amestrisian alchemy derives its power from the earth's crust, whereas Xingese alchemy uses the "veins of the dragon," basically a concept that's sort of like a pool of spirit/life energy, which is why it is so deeply connected to medical applications. And it is also heavily implied that the Sage of the East is Father, while the Sage of the West is Hohenheim, and these two individuals brought alchemy to Amestris and Xing, respectively, from Xerxes.