The problem with AAC format is that it involves various types of compression such as LC (low complexity), HE (high efficiency) and AAC+ or HEv2 and because of that it requires careful implementation and testing when included in third-party software. If you look at MCX8000 and Engine Prime specifications sheet it only states a generic AAC format compatibility at different bitrates, but doesn’t specify exactly what types of compression it can handle so that user can prepare their files format in advance. Given the fact that Engine Prime allows you to manage an iTunes library, we can safely assume the software verifies the id3tag of a file to establish it’s compatibility. Usually, variable bitrate audio files downloaded from iTunes are encoded to 256 kbps (Low Complexity) so when Engine Prime software verifies AAC id3tag and sees a bitrate higher than 256 kbps for a VBR file it will dismiss the file as incompatible. Now it is up to Denon to modify Engine Prime verification process to accept AAC files encoded at a variable bitrate of 320 kbps.