Francois, the beginning of your second post answers the question you had at the end of the first. As channel 6 is right below the FM frequencies, any EMI(electromagnetic interference)which affects one will also affect the other.
Your description indicates that this isn't a ground loop problem, or at least that it isn't the major cause. Switching class D amplifiers which turn on and off several hundred thousand times a second generate enormous amounts of EMI and need careful design to minimize the effect of this. The EP350 trouble shooting page you link unfortunately seems to indicate that it's up to the user to try to do this, by using a heavily shielded coaxial cable to a distant antenna and by not using an indoor(e.g., folded dipole)antenna nearby.
Keeping the sub and its power cord as far as possible from the receiver and TV connections can help.
