Hi, I have the same problem with the standalone version of SoundID Reference.
I think there is no way to put DigiCheck NG before SoundID Reference, because SoundID Reference acts as the "audio driver" which directly gets feed with the audio stream from the computer and then distributes its calibrated signal to the RME software / hardware.
In the DAW (Nuendo 12) I'm using the workaround by sending my master bus to a second bus / output, which I then select in DigiCheck NGs "Input Selection...".
I also tried Decibel from Process Audio. With this metering software it works, because you can choose Decibel as the main audio driver and then route the output from Decibel to SoundID Reference, which then does its magic and feeds the RME software / hardware. Another cool thing with Decibel is, that you can use a tablet with the Decibel App and have an extra screen for monitoring.
Would be nice if this would be also possible with DigiCheck NG.
Win10 Pro 64bit v.1903 | Nuendo 10 latest | i7-4820K 3.7GHz | 32GB DDR3 RAM | RME Fireface UFX+