Hello Fish,
I have the same problem. I am so glad it is just not me. If you have a good ear as most audio people do you can hear this. Church it is not the device. When I airplay from my office, to a bose portable with APeX, and an apple tv gen 3 or gen 1 you can hear a slight delay with the apple tv the most. For me all speakers are on the same floor so I am about 20 ft from each speaker and it sounds like an echo, right fish? I even tried to swap out "devices" and it is still the same thing.
I bought all Bose the "Best type A Device" to solve this but still a problem!???
Fish, the best way to "solve" this the way I found it. get all "devices" playing a song that it happens the most with. I found most pitpull, LMFAO, or even something with a steady beat. Play the apple tv as loud as you need it to be first. Then raise the sound on your other device slightly louder then the apple tv. This also works in reverse too. APeX and my office does not matter, they sound great together. I also have the gen 1 Apple tv and the same thing happens with it. I moved to a larger house and wanted the "Apple airplay" party but became a bust with this problem.
IF there are others out there PLEASE post here to get this solved. Apple knows this because it also happens at the store!!!!!