How do you configure Blackhole to play 7.1.4 on Apple Music?

Can someone please tell me how to configure Blackhole so Apple Music will play atmos in 5.1.2 (or 7.1.4 or anything above 5.1) for a soundcard with >16 output channels?

I've tried

creating an aggregate device in Audio Midi with Blackhole and my soundcard (Focusrite Clarett) and setting that as the system sound output, configuring the speakers (in AudioMidi) to route to the Clarett output channels - doesn't work: I get 5.1.
creating a Multi-Output Device in Audio Midi with Blackhole or Built-in Output and my soundcard - doesn't work: I get 2.0

I don't fancy forking out $100 for Loopback to fix something that really should work! Can anyone help?