none of the speakers have any cut off
it is the way the amp is made to hadle only the original speakers which are full range also
they are very efficent, meaning they use little pwoer for their output
when you changed the speakers they now have less output and different audio frequencies
causing you to think it is not playin the full rnge but it is just not giving the same out put of the other frequencies
the front speakers are connected in wiring for the tweeters as well
so they create a 2 ohm line instead of the usual 4 ohm as the system is set up to handle this