Why use one amplifier, when you can use two for double the price?

Of course I'm talking about bi-amping - the method of driving a single two way loudspeaker from two amplifier channels.  Most PA Loudspeakers fall into the the following categories:-

• Passive
• Powered (some manufacturers like Mackie call this Active for some reason)
• Bi-amp / Tri-amp Active

Passive speakers use an internal crossover, and are driven from a single amp channel.  Powered speakers have an amplifier (or sometimes more than one amplifier, and other electronics) built into them.  Bi-amp / Tri-amp active speakers have no crossovers built in, and need a separate amplifier for each driver (woofer / tweeter / horn) inside the speaker.

Back of an envelope - Passive vs Bi-amp design

So what's the big advantage of bi-amp speakers, then?  And is it worth shelling out on double the number of amps to use them?

It's a bit techy but basically passive crossovers sap power from the amplifier. They also have very shallow cross-over slopes of around 12dB per octave. That allows a significant amount of range where both HF and LF drivers are radiating the same sound, which can create comb filtering or other bad effects and phase problems. An active crossover allow for much higher crossover slopes of 24bB per octave or higher, so you're getting less overlap between the drivers.  There is also no loss of power to the internal passive crossover.  You'll gain about 3bB in headroom or output, possibly more in some cases.
When amplifiers are connected directly to the drivers, and not through a passive crossover, they have a greater ability to control the speaker cone, and so reduce the possibility of ringing and other negative phenomenon.
So, maybe it is worth converting your current speakers to bi-amp, and buying more amps and active crossovers?