The impedance curve of the speaker is separated into two pieces: high and low. The amplifiers only get one of those respective load curves. With most bi-amp-able speakers, the low cutoff feeding the midrange and tweeter is still in the circuit. Likewise, the high-cutoff still rolls off the highs hitting the woofer. Put both together as one load to one amplifier, and additional interactions affect sound quality, usually for the worse. Bi-amping worked very well for my system, but ymmv.
