konradh
1. Sing the vocal twice.
That we have been told is apparently not an option.
But if there is a chorus sung identically elsewhere, you could copy them under each other and try Vocal Sync.
Leave verses undoubled for variation.
But what I would really like to know is what was causing the effect?
If you go close to the speaker that is apparently "dead", do you then hear something?
I have heard recordings with this effect using two pipes made of concertina-like material, shortening one and lengthening the other. But that only worked with headphones or being right in between the speakers