Sounds like something is in the room is coupling at the wavelength of your B (and certain harmonics of it).

As a step - plug in earphones so your monitors/monitor stands/keyboard stand/desk/etc. are not connected to the audio output. This will isolate anything that may be coupling. The hypothesis, if this is a coupling issue, would be your earphone isolation would not reproduce what is thought to be sympathetic vibration of something else.

If that clears things up then you need to look at isolating the vibrations of your monitors from anything else in the room. Sometimes it can be something not bolted down "flapping in the wind". Sometimes it's right at the monitor interface to its supporting structure.

j'ai fais un teste avec des Adam ax5 et le problème est encore présent ,tout est régler sur le neutre ,
je comprends pas que certaines notes ,le si et le do sonnent si fort.
,les fréquences sont trop élever ,j'ai tester avec des écouteurs et la aucun problème.

le SAV n'a pas de solution et d'explication a ce problème ,déçu.
en plus c'est désagréable de jouer au piano avec le si do.

Google Translate:

I did a test with Adam ax5 and the problem is still present, everything is set to neutral,
I do not understand that certain notes, the B and the C sound so loud.
, the frequencies are too high, I tested with headphones and the no problem.

the after-sales service has no solution and no explanation for this problem, disappointed.
in addition it is unpleasant to play the piano with the B C.

It definitely sounds like a kind of audio feedback (build up of acoustic frequencies that emphasize a particular frequency area).

If it is not in the Phones, it is not a problem with the synthesizer.
Are any open microphones in the area?
Look for any potential feedback loops.
What else is connected?
Anything connected to the MONTAGE A/D Input, is there an open built-in mic on a computer?
Disconnect any connection to computer, or other audio device. Eliminate as much as you can to simplify your setup during the troubleshooting process. If you need more assistance, it might help to have a bit more information

This much we know the audio sent to the MONTAGE Main L/R is exactly the same as goes to the Headphone output. The fact that the signal is clean in the Phones clears the synth tone engine.

rien n'est brancher en USB ,midi, rien que mes monitoring au montage,
le problème c'est ce do si qui ont une fréquence trop éleve, c'est pas autre chose ,peut être un défaut .
j'ai essayer un autre montage en magasin ,le problème est présent .

est ce qu'on peut traiter le problème par une table de mixage ?
ou est ce un défaut sur tous les montage !

la video

I don't have this issue with my Montage using HS8 monitors sitting on a wooden platform with my particular room acoustics, keyboard stand, light fixtures, etc.

We seem to have a disconnect here, though. Information as to the likely nature of the problem has been communicated but not accepted.

I would be attempting to isolate those monitors from the environment to see if the monitors have this frequency response themselves (for some odd reason - I doubt it) or if it's something in the room causing the issue. In which case, running the speakers with long cable (careful, though - balanced would be best) to the next room may be able to do this.

Headphone tests appear to indicate the keyboard isn't generating the issue. So it's something else (environmental).

Speaking of long cable runs - it's interesting that B natural is close to 60Hz which is an AC frequency here in the states. France it's 50Hz and Canada is 60Hz like the US. Not that this means anything - but long runs and the wrong kind of interference (i.e. aggressors) can cause issues. I admit, the correlation to frequency is a bit of a reach, BTW.

merci pour tes réponses Jason,

le problème est bien du montage ,j'ai tous essayer ,en changeant d'enceinte ,Adam ax5 ,Focal alpha 65 rien ne change,
je n'est pas de problème avec le courant de ma maison ni avec l'environnement de ma maison.

j'ai un cvp et j'ai eu un genos ,aucun problème de son.
ca doit être un défaut de fabrication, je ne vois pas autre chose.
merci de ton aide eddy