phase cancellation - volume

Hi,
Once again a newbie question... I have recorded some interviews with a shotgun Sennheiser connected to my Nikon D800 via an home made XLR to stereo jack... working fine so far except a little background noise.
I realised that although the sound plays perfectly well on distant stereo speakers... it becomes bad or silent on smaller device like my cellphone Note 4... then I discover a problem of phase cancellation.
Following the very easy solution that FCPx offers, by ticking phase left or right only in the gain filter... the phase cancellation problem is solved.
BUT my problem is that the sound is now by far too low to hear, although the meters seem unchanged. If I export the video, the sound is still too low.
In order to have the correct voice, I need to push up the gain by at least 8... but then the meters often go in the red.
Another issue is that for some voices, doing the phase left seem to reduce the sound quality.
What is the best way to go around that?