Thanks, mate. I come with very interesting findings. To me anyways.:
No matter what I tried, I always had a very quiet front/center output when playing back my 4.0 BD in 5.1.
Now, I have found out why: In my Oppo, the surround setup under Audio Processing-Speaker Configuration has always been FL/FR and SL/SR without a Center channel, since I don't have one. And it has worked flawlessly so far.
However, just out of curiousity I changed the setting and added a Center channel in Speaker Configuration, and voila, I had the exact same sound as I get it when I play back my 4.0 BD without a Center channel.
I then put in a regular BD again with a 5.1 DTS-HD MA track (Big Bang Theory, as it has mostly dialogue, and with the center channel employed in the speaker configuration setup, the dialogue is way more quiet, while without it, it's perfect.
It's strikes me as odd, that I'd have to change the settings just for my BD to get the right amount of volume, when normally there isn't a problem at all. What does that mean? And most importantly: How can I remedy that?
Now to the procedure:
I first opened the Cubase project of the song I wanted to do the testing with, muted the stereo output channel, and made a downmix into two mono files, which I then imported into Adobe Encore, and without muting, since there is no sound to begin with, assigned those to the Center Mono only, and LFE mono only channels. After which I created two downmixes: One with 6 mono files, and one interleaved file containing all 5.1 channels.
I then changed the names of the mono files according to Cirlinca's requirements, and opened Cirlinca, where I put the interleaved file, which was immediately recognised as 6 channels, and one of the 6 mono files, and assigned 6 channels to it.
(Interestingly, doing 5.0 doesn't seem to be accepted. I threw away the LFE channel of the six mono files, but that didn't help either. It just won't do 5.0, even though it's an option. Weird, especially since 4.0 isn't a problem.)
I burned them onto a RE-BD, and gave it a spin. It worked in both ways with the Center Channel activated in the Speaker Configuration. The LFE channel doesn't appear to make a difference. It works both ways (On/Off).
After having used new Cubase files successfully, I figured I should give the Adobe Encore mono files a shot, and duplicated + muted two mono files, assigned them to their respective channels, and got the same results, which brings me to the conclusion that it was the center channel in the Speaker Configuration setup all along.
Now, I still don't know what this sounds like on a regular 5.1 setup (A/V receiver), nor do I understand why it only works this way: Shouldn't the 5.1 downmix deal with it automatically, just like with other movies. The only difference is that there isn't any audio in the Center to begin with, so why does my Mahavishnu SACD not have the same problem?
To summarize:
5.1 mixes of the 4.0 mixes with a dead Center and LFE channel works now, but only if I change to 5.0 in my Speaker Configuration setup on my Oppo player.
Whether I add two muted mono tracks from duplicates in Adobe Encore, or make a downmix of the Stereo Out in Cubase, which are then imported into Adobe Encore, doesn't seem to make a difference.
Only the Center channel changes the volume level, whereas the LFE doesn't affect anything (or not that I had noticed).
I can't do 5.0 in Cirlinca, although it's officially an option given.
There is no difference in one 6 channel track, or 6 mono tracks, the former being the way to go for simplicity sake.
I have yet to hear it on a 5.1 system.
What do you guys think?
Any ideas?