Someone told me he had the same problem and he just took out the 6 voice cards and put them back, calibrated the beast and everything worked great. So that's what I did, well what I wanted to do because Mr P6 is playing with me or I might have scared him by just opening it and having a look inside because since then, all voices works great...
So I will keep an eye on it and will run tests often to check that every voice still sing great!
Thanks to all