Looking at the 5.1 in from HD621, the rear panel with sub left, sub right and sub, and 3 sub blurb, I really see a focus on 5.3 for the 8 channels.

Allowing for great Trifield stereo with monitors and subs and 5.1 surround.
An appealing proposition.

Meridian G68 AXV, HD621 | Cinepro 2k6mk5 | 4x Scanspeak A4 monitors, center & 26W Sub | RPi Hifiberry digi+ | Roon