Moon 430HA connect to D400

[hardcore]

Thank you for your answer,
I will do it both ways and I will check which one sounds better, in theory one channel of each of D200 should sound better than all the preout in the master.

This may not be the case - right now your Master & Companion are isolated and the only cable connecting them together is the RCA-RCA link.

If you connect your headphone amp to both amps' RCA sockets, this will 'join' the Master & Companion grounds together at your headphone amp, which creates an extra ground loop. Ground loops will introduce more noise and affect the sound.

It's a similar concept to why REL recommends using two separate subwoofers if you have a dual-mono amplifiers. You want to avoid coupling the ground planes of both amps together through another device (like the subwoofer or the headphone amp in your example). Not directly related but some explanation on isolating the grounds in the link below (ignore the high level connection part!). The Devialets in dual-mono mode are operating as balanced differential amplifiers.

https://relsupport.zendesk.com/hc/en-us/...ks-Part-II