Which codec and format are Audible downloads?
As this post over at the apple stackexchange says, .aax files from Audible are indeed encrypted m4b files, which means that if I would decode an .aax file without converting it, I would get a .m4b file.
This is possible with the following command:
ffmpeg.exe -activation_bytes XXXXXX -i input.aax -c copy output.m4b

The -c copy ensures that the data will not be converted.