I'm reminded of the Sumo preamp thread a few days back that had a link to a Dagogo interview with Roger Saunders:
http://www.dagogo.com/an-interview-with-roger-sanders-of-sanders-sound-systems
Here's the first paragraph of his thoughts on crossovers:
Passive crossovers have dreadful performance. This is another extensive topic that I cannot address adequately in this interview. So just allow me to summarize by saying that passive crossovers have the well-known problems of phase shift, hysteresis losses, group delay, inadequately steep crossover slopes, and distortion. Less appreciated is the fact that they insert inductors, capacitors, and resistors between the amplifier and the speaker’s drivers. This isolates the woofer from the amplifier and prevents the amplifier from having tight control of the driver. Or to put it another way, the damping factor of the amplifier is degraded by passive crossovers.
There's more to read after that.