Hey mate,
the DB25 on the AXR4 carries digital AES/EBU signal.
[AES/EBU] connector - For connecting to devices that input or output digital signals in the AES/EBU format. You can select between the [AES/EBU] connector and the 7 OPTICAL B [IN]/[OUT] connector for use. The jack and format in use can be set on the DIGITAL I/O MODE screen on the AXR4 device or on the Settings screen on the dspMixFx AXR.
Is Delta’s DB25 carrying AES/EBU? I have no clue.