Se e' 7.1 sono canali discreti effettivi.
Alle tracce puoi applicare elaborazioni o meno (direct/pureaudio/surround).
Sul manuale , effettivamente in modo un po' criptico , c'e' scritto qualcosa e qualche consiglio dove usarli ... almeno sul mio 875.
Poi sono sempre DSP per cui "non stai piu' ascoltando l'originale" ma qualcosa che a te piace di piu'

(una volta che decidi di usarlo)
Magari con alcuni DSP c'e' il rispetto del segnale originale sulle altre casse ... ma e' cosi' importante a questo punto ?
Per esempio , ce l'ho in inglese qui':
• THX Ultra2 Cinema
This mode expands 5.1-channel sources for 7.1-
channel playback. It does this by analyzing the composition
of the surround source, optimizing the
ambient and directional sounds to produce the surround
back channel output.
Neural THX 5.1/7.1
Neural-THX Surround employs psychoacoustic frequency
domain processing, which allows delivery of a
more detailed sound stage, with superior channel separation
and localization of audio elements. The Neural THX
5.1 and Neural THX 7.1 modes can expand any 2-channel
stereo source for 5.1- or 7.1-channel playback,
respectively. Use them with CD, radio, cassette, TV,
VHS, DVD, and other 2-channel stereo sources, including
video games. Neural-THX Surround can also be
used by broadcasters to encode and transmit surroundsound
content over a stereo signal, which listeners can
enjoy as either surround sound or normal stereo. XM
Satellite Radio, for example, is using Neural-THX Surround
on select channels, which the AV receiver can
expand from 5.1 channels to 7.1 channels.