It looks like the S/PDIF output might do what you want if you want to use that for now, but it won't fix your problem really as you won't be able to have DTS-HD and TrueHD over S/PDIF.
I don't know if the kernel module will enable HDMI audio or not (or if it's even possible to build the kernel module for LE).
17:03:16.846 T:139651876660480 NOTICE: Device 7
17:03:16.846 T:139651876660480 NOTICE: m_deviceName : @:CARD=USB33900DHE,DEV=0
17:03:16.846 T:139651876660480 NOTICE: m_displayName : Plugable USB3-3900DHE (@:CARD=USB33900DHE,DEV=0)
17:03:16.846 T:139651876660480 NOTICE: m_displayNameExtra: Analog
17:03:16.846 T:139651876660480 NOTICE: m_deviceType : AE_DEVTYPE_PCM
17:03:16.846 T:139651876660480 NOTICE: m_channels : FL,FR,BL,BR,FC,LFE
17:03:16.846 T:139651876660480 NOTICE: m_sampleRates : 48000
17:03:16.846 T:139651876660480 NOTICE: m_dataFormats : AE_FMT_S16NE,AE_FMT_S16LE
17:03:16.846 T:139651876660480 NOTICE: m_streamTypes : No passthrough capabilities
17:03:16.846 T:139651876660480 NOTICE: Device 8
17:03:16.846 T:139651876660480 NOTICE: m_deviceName : surround21:CARD=USB33900DHE,DEV=0
17:03:16.846 T:139651876660480 NOTICE: m_displayName : Plugable USB3-3900DHE (surround21:CARD=USB33900DHE,DEV=0)
17:03:16.846 T:139651876660480 NOTICE: m_displayNameExtra: Analog
17:03:16.846 T:139651876660480 NOTICE: m_deviceType : AE_DEVTYPE_PCM
17:03:16.846 T:139651876660480 NOTICE: m_channels : FL,FR,BL,BR,FC,LFE,SL,SR,UNKNOWN1,UNKNOWN2,UNKNOWN3,UNKNOWN4,UNKNOWN5,UNKNOWN6,UNKNOWN7,UNKNOWN8
17:03:16.846 T:139651876660480 NOTICE: m_sampleRates : 48000
17:03:16.846 T:139651876660480 NOTICE: m_dataFormats : AE_FMT_S16NE,AE_FMT_S16LE
17:03:16.846 T:139651876660480 NOTICE: m_streamTypes : No passthrough capabilities
17:03:16.846 T:139651876660480 NOTICE: Device 9
17:03:16.846 T:139651876660480 NOTICE: m_deviceName : iec958:CARD=USB33900DHE,DEV=0
17:03:16.846 T:139651876660480 NOTICE: m_displayName : Plugable USB3-3900DHE
17:03:16.846 T:139651876660480 NOTICE: m_displayNameExtra: S/PDIF
17:03:16.846 T:139651876660480 NOTICE: m_deviceType : AE_DEVTYPE_IEC958
17:03:16.846 T:139651876660480 NOTICE: m_channels : FL,FR,FC,LFE,BL,BR
17:03:16.847 T:139651876660480 NOTICE: m_sampleRates : 48000
17:03:16.847 T:139651876660480 NOTICE: m_dataFormats : AE_FMT_RAW,AE_FMT_S16NE,AE_FMT_S16LE
17:03:16.847 T:139651876660480 NOTICE: m_streamTypes : STREAM_TYPE_AC3,STREAM_TYPE_DTSHD_CORE,STREAM_TYPE_DTS_1024,STREAM_TYPE_DTS_2048,STREAM_TYPE_DTS_512
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