Moderators: MattKingUSA, khz
I did as suggested, and here's what happened:
After renaming the .conf file, the Audio Master Out worked just fine.
On loading my project, and taking care to remove previously connected busses, things went sour, just as before.
I really don't know if there's any sort of corruption in the file...
Maybe, just maybe, I might just redo everything, or even start a new project from scratch, and see what transpires.
Thanks a whole lot for bearing with me.
OK, done!
Everything is working as it should, finally!!!
A million thanks for your guidance 
after all this mess, you must have had Options... > Plugins > Instruments > Dedicated audio outputs turned on; which enables plugins > Audio > Dedicated on all MIDI tracks and buses, including the MIDI Master input and output buses;
you also had "Auto Connect" on all of those dedicated audio outputs too, that is/was the reason why you heard the whole song on the speakers on normal playback;
trouble on export is/was that NONE of those additional, dedicated audio ports appear on the the Track > Export Tracks > Audio... export dialog as output options for mix-down, so none of the MIDI instruments are/weren't rendered to export at all.
hth.
I checked, and rechecked, before and after getting rid of the Qtractor.conf file. I checked the busses, the options, and went through the tabs within the Options dialogue box. Nothing. Because I suspected exactly what you referred to. But nothing.
Believe me!
Hi, maybe you didn't understand that the (global) options are just an indication for current and new sessions onboarding... sessions that once saved on that board will keep their previous settled configuration even still you turn it off later; the clue is on each track and bus plugins > Audio > Dedicated setting I've been telling you to get off about from the start, remember?