Most energy is required in the midrange, followed by midbass and bass. There's less power requirement for the HF amp in a bi-amped system because there's generally less signal content. Also, because the HF signal isn't riding on top of a LF signal, removing the LF component makes less of a peak-to-peak voltage requirement too.Rod Elliot has written a pretty good article that describes some of the issues. Check it out: