If I encode a file with HE-AAC and ReplayGain it the file will play +6 db louder than the original which is also ReplayGained. Both files are scanned with track gain and playback is also on track gain.

Foobar only plays too loud on MONO AAC files. Foobar thinks they are stereo according to properties. This must be why it sounds +6 db or twice as loud compared with the original mono wav.

PS
Mono files in general seem to play -6 db in Foobar. This seems weird.

Problems related to AAC :

Foobar decodes 6ch AAC as 8ch.
The channel order of multichannel AAC is wrong.

foobar2000 isn't the only program to mis-identify mono AAC/M4A audio as stereo; the FAAD2-based Winamp plugin hosted on RareWares does so, as well. Strangely though, the latest versions of TCPMP do properly identify the audio as being single-channel.

Apparently, if it's a libFAAD-related problem, the error has been corrected somewhere along the way, and the sourcecode simply has not yet percolated to all the appropriate outlets.

- M.