In my system, Apple Lossless sounds great running over my AppleTV through my processor. I've compared it very closely to my Oppo BDP-83 and really can't notice a significant difference, and the Oppo is no slouch as a transport.
However, it sounds like you're using AirTunes over an Airport Express. I'm 99% sure that when you stream over AirTunes to an Airport Express, the Mac converts to a lossy format on the fly for AirTunes to decode.
However, it sounds like you're using AirTunes over an Airport Express. I'm 99% sure that when you stream over AirTunes to an Airport Express, the Mac converts to a lossy format on the fly for AirTunes to decode.