"But if I do as suggested it only loads the 'AUMatrixMixer'."
There's probably something wrong in your 'aucache.db3', located in "Max 5 Preferences Folder".
In [p misc], click the message called 'resetcache' at the bottom of the patcher to rebuild this db3.
"DLSMusicDevice does not appear in the menu"
You're right: remove the extra arguments 0 2 before the name to only get [audiounit~ DLSMusicDevice]:
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