[DaveBlake]

One good side on keeping replaygain in the db would be to be able to check quickly which files have or don't have replaygain in your library.

But is that ever likely useful as a smart playlist criteria? Only use I can think of is as a "what files did I forget to tag with gain?" check, and that is not really a media player task. There are so many other things to add that are wanted as filter criteria to aid navigation of large music collections and improve music choices, that replay gain as data seems to me just clutter.

right. but it still doesn't make sense that it sits in the codec. then it should sit in the decoder frontend.

I agree it doesn't make sense, then again I know nothing about the AE architecture so not in a rush to meddle.