I used a tiny SBC and a 7.1 channel USB soundcard to upgrade a 15 year old Yamaha AV 5.1 "Dolby Digital" receiver (with toslink/spdif) to handle the latest codecs. The old receiver has a "6ch input" selector that doesn't do any decoding - just takes the 6 inputs, amplifies them and sends them to the speakers. I also use the extra 2 channels from the USB 7.1 audio to a 2nd stereo amp that powers the adjoining room. (This involves pulseaudio channel mapping).
It took about 2 days to make sure that absolutely everything worked, including turning stereo into 5.1 + 2, and adding LFE to the adjoining room speakers so they didn't miss out on multichannel bass. Technically, I could map the extra channels from a 7.1 or 9.1 source into their nearest 5.1 neighbors, but I left that part disabled because I don't have any sources with that many channels.
The most tedious part of the setup was proving that each source type was supported, and all the speaker mappings were correct. Test files are extremely useful and there's no one source for all of them. I spent quite some time finding these files for myself.
It took about 2 days to make sure that absolutely everything worked, including turning stereo into 5.1 + 2, and adding LFE to the adjoining room speakers so they didn't miss out on multichannel bass. Technically, I could map the extra channels from a 7.1 or 9.1 source into their nearest 5.1 neighbors, but I left that part disabled because I don't have any sources with that many channels.
The most tedious part of the setup was proving that each source type was supported, and all the speaker mappings were correct. Test files are extremely useful and there's no one source for all of them. I spent quite some time finding these files for myself.