Encoder bandwith and low pass filters
Just as in Fraunhofer encoder wiki, it's advisable to know apple encoder's default lowpass filters with bitrates, to be able to change defaults with --lowpass command option. Low pass filters is important to ascertain how the encoder chooses to omit high frequency information.
FFmpeg with AudioToolbox
In recent versions of FFmpeg on OS X it is possible to use the Apple encoders via AudioToolbox. AAC encoder is called
aac_at. Unlike afconvert, it preserves metadata when encoding.