Anyway......
After a bit more research it seems the
info in the post filler56789 linked to is correct, only maybe it's been interpreted backwards.
The conclusions was:
"As already expected, most encoders and decoders use only the "implicit channel mapping"".
However I think it's exactly the opposite. They'll mostly only use the "explicit" channel mapping, because the "implicit" channel mapping implies 3.1ch AAC encoding is possible. The "explicit" channel mapping however only includes a pre-defined set of channel mappings.
The info in the above link refers to "16 available PCE’s" to define channel mappings. The first seven are listed here:
http://spectralhole.blogspot.com.au/...nel-model.html
Just as I thought I was onto a logical explanation it occurred to me there seems to be only one official PCE for 4ch audio, and that's for a 4ch layout consisting of front stereo, front centre and a rear surround channel (the 4.0ch layout mentioned in my earlier post), yet QAAC will still accept and encode the two 4ch QUAD mappings, but not 3.1ch...... there must be a logical reason for that.