Digital Coax 1.5m

[devianet]

Worth bearing in mind that the companion has the ability to output a mono (downmixed from stereo) subwoofer output. This is exactly how I have my 440 configured, to drive my REL at line level.

This can only mean that the digital link between the two units is stereo. So, in my configuration, the master takes the R signal and uses it for both amp channels in that box (left amp channel inverted). It then sends both R and L to the companion. The companion takes the L signal and uses for both amp channels in that box (left amp channel inverted). Then it also takes the L + R signals and mixes them to mono for the subwoofer output.