I've tried both but currently it's HD621 before my XS.

It used to be the other way around but I had problems with one source going to my Lumagen. In addition the HD621 has 6 HDMI inputs as opposed to 4 on the XS

Main system - Sources (Kodi-HTPC, Vero 4k, FireTV, Sat) into Trinnov Altitude 32 (8-16) (+Roon), Trinnov Amplitude 8 amp & Vivid Audio (Giya G3, V1.5) and REL Stentor III speakers. Others: F80 + Raspberry Pi and other Pi systems.