Hi,

I don’t know if it’s any help, but the UHD722 comes with a Din Comms to SL comms adapter, so I wonder if you could go from the slave speaker SL out back to the din comms on the HD621.

Just a thought

Brian

Main: G65, MD600 Sooloos, MC200 Roon Core, MS600, 210, 218, UHD722, DSP5200SE, DSP5200HCSE, M6 & DSW.2
System 2: AC200 MS200, WiiM Pro+, M6.
Other: DSP5000 (96/24v1), DSP3300.