Ok. I suspect my Oppo issue is loser error on my part (it seems that the RS meter doesn't go below -10 on its dial!) and I bet that Oppo + Discrete will turn out to be -15dB.
In other words, the Oppo 980 is applying the -5dB adjustment on its analogue sub output even when the main five speakers are set to "Large". A bit annoying, but I'll be happy if that is the explanation.
Hopefully, many of us will be using HDMI soon. However, I suspect that the Oppo will also apply a -5dB offset to the LPCM sub channel.
This -5dB offset is going to be player dependent and is something for everyone to watch out for!!!
Can you boost the sub channel in the Oppo's channel trim settings? Does that fix it?
If I can find my 5.1 setup disc, I'll test this myself.