Luc, as Mark Johnson pointed out, don't just switch off the sub and leave the other settings the same, because the receiver will still be sending output to the sub which can't use it. Set the bass to "NONE", not "BOTH". The 80HZ crossover will then roll off the surrounds and center below 80Hz and send it to the M22s. Don't worry about any type of direct setting, you want bass processing. The M22s aren't bass powerhouses without the sub and you didn't say why exactly you're trying them like that, but mine give pretty strong output at 50Hz without a sub and much weaker but still usable output at 40Hz.
