I'm not sure what exactly the difference is between fields date added and Date added (album). However, this is my understanding that both are MB internal as opposed as Date modified which appears to be the System Modification Date for the file on disk.
That would be great to have the System Creation Date field in MB for sorting. If for some reason, files are removed and re-added to the library, the original date added is lost. The Creation date would be great to have because this date never changes.
Thanks and keep up the good work, Musicbee is awesome !!