5 Stereo and 7 Stereo will be more bass heavy because of the way the signal steering takes place. It will also be louder at a given volume setting, although the overall effect is not quite as pronounced as for the subwoofer. You are copying left channel (without any decrease in signal level) to at least one extra speaker, copying right channel to at least one extra speaker, and copying a mix of the two to the center. Then you are applying the normal bass management on all of those speakers, such that you get two and a half copies of the bass from each of the two original channels.