It would be best to check your manual for the sub, but the outputs will either completely bypass the sub circuitry, or be fed only what is not filtered out. If this is the case, then your speakers will have more headroom, as they won't have to work so hard with the lower frequencies. With the sub 'Off' you might get an automatic full frequency bypass, best to check.
On the back of the BM5As there are frequency cutoff switches which you can use to cut out frequencies lower than the switch setting, this is used when you integrate the sub into the system.