Normal impedance for car-stereo is 4ohm. For hifi home stereo is 8ohm (and 12ohm for some tweeters).
The bose stereo is on 1ohm, to minimise power consumption and produce loudness from a mini-amplifier. However, quality is sacrificed. This is why the standard Bose system is using a very small amplifier for all those speakers.
If you are about changing speakers, it is better to take the whole amplifier off the new speakers and plug them straight in the new stereo, or the 4ohm speakers that you have just got will be always thirsty for more power (4 times more) that the poor mini-amp does not have.