this is out of experience...
If you drive a lower rated spk with a higher rated amp,
eg a 6ohm spkr with a 8ohm amp, the sounds seem to be deeper and softer
While if you use a lower rated amp with higher rated spkrs,
eg a 4ohm amp with 8ohm spkrs, the sound is really hard, and the base is stiff and knocking, its also slightly distorted
Just a few things ive noticed over the yrs