Is bi amping worth it ?

an interesting thread. as others have posted. Bi-amping has a lot of plus and minus and it all depends on what you are targeting to solve or improve. 

for me, I had the weird idea of biamping the ATC speakers. what I did was to use the X150 to drive the mains and XA25 to drive the tweeters, 

I personally did not find a major improvement, there was some change, I would like to point out a better dynamic, but not enough to justify the extra cost of an amp.

YMMV though ..

Very good explanation russbutton!  I started my multi-amp journey with a Lafayette integrated amp and a homemade Sanken amp using a couple 30watt hybrids.  As mentioned previously, two key points are the electronic crossover to limit the amplifiers demands and eliminating the passive speaker crossover parts entirely.

@billpete  Active crossovers and equalizers are totally different animals.  Let's first understand the difference between active and passive crossovers.  A passive crossover is an electronic circuit that has no power supply.  It is just a circuit that sits between the output of your amplifier and the loudspeaker drivers.  

An active crossover has a power supply like your preamp and power amp, and sits between your preamp and multiple power amps.

An equalizer is a device with a power supply, that typically is located on an extermal processor loop on your preamp, or a tape recorder loop on your preamp. 

The do very different things.  An equalizer alters the system response.  Typically the operate on ten bands across the audio spectrum.  

A crossover, either passive or active, is designed to separate out the signals being sent to the loudspeaker drivers.  High frequencies for the tweeter and low frequencies for the bass driver.  If you have a 3-way system, then the crossover has the added function of allowing only an intended band of signals for the mid-range driver.

Most loudspeakers use a passive crossover.  The user has a preamp & power amp, or an intergrated amp or receiver, and run one speaker line to the speaker.

In a system with an active crossover, the preamp sends a line level signal to the active crossover, which then outputs line level signals that have only those frequencies suitable for the given driver.  If you had a 3-way system, then on each side, the active crossover would have 3 line level output signals for 3 separate amplifiers, each of which powers a single driver.

The more expensive amplifiers are very good at managing the very complex impedances typical of loudspeakers with passive crossovers.  

Less expensive amplifiers can do a very good job if they are only powering a single driver and do not have to cope with the complex loads that a passive crossover have.

I run a set of Linkwitz Orion loudspeakers, which have an active crossover.  I use a B&K AV1260 power amp which has 12 channels at 60 watts/channel.  Each side of my speaker has two tweeters run in parallel, a mid-bass driver and two 10" bass drivers.  There is one amp channel for the tweeters, the mid-bass driver and each of the 10" bass drivers.  B&K made MOSFET amps that were well thought of in their day, but was never seen in the same class as Mark Levinson, Bryston, McIntosh, etc.  Being that my amp channels don't have to deal with a passive crossover, they do a very good job.  I *LOVE* my system.

@russbutton Nice explanation, thank you. So, why is a Krell amp better suited to run a 3 or 4 way system with passive crossover? I believe that my Parasound amps do a very fine job of powering the AR9's. They are 4 way with 2x12 wooferrs each on the low side, the top is made up of a low midrange driver (8 inch, I think), upper midrange driver and a tweeter. There are 3 toggle switches in each cabinet, allowing control of bass, midrange and treble. You can set at zero, minus 3db or minus 6db to adjust mostly for room acoustics, I would imagine. I currently have them all at zero.

I am not in any way questioning your answers, just trying to understand why. BTW, good job explaining to someone with very limited knowledge of the components. I do understand the basics but most of the EE stuff is well over my head. Thanks again.