Why does bi-amping speakers sound better?

I see a KEF in your future...

The very best design would be to put a DAC in the speaker with the amp, and just send control signals to it.

not sure the realization is quite there yet

Realization is quite there already : AVI ADM9.1 already have Wolfson DAC's in the speaker.

Agree that active loudspeaker is the next logical step.  There are a large number of fully active loudspeakers that accept a direct digital input signal (B&O, Meridian, Dynaudio, Genelec, Avantgarde, Focal, etc.).

I am 90% sure which brand of amp I want to get, just trying to figure out if I should bi amp or not.

as a example should I go with

two 2 channel amps at 400w x 2 so each speaker would get 400x2

or should I go with a single 800w stereo amp so each speaker sees the same 800w, just with one channel of a amp

According to this datasheet the "power rating" of the Kappa 8.1 is up to 300 watts. So an 800 watt amplifier would presumably be overkill, as would two 400 watt amplifier channels per speaker. Also, I’ve read that the 8.1 is not quite as current hungry as the Kappa 8 or Kappa 9, which if I recall correctly have notoriously low impedances in the bass region.

Given that, as well as the 6 ohm nominal impedance of the speaker, it seems to me that the best and most cost effective approach in this particular case is likely to be a single stereo amplifier having an 8 ohm power rating in the area of 300 to 400 watts, and a 4 ohm power rating of double those numbers or close to it.

Keep in mind that part of your amplification dollars goes toward watts, and part goes toward sound quality. Therefore paying for watts that won’t or can’t be used may misdirect some of those dollars.

But to answer your more general question:

In the case of passive biamping (i.e., without an electronic crossover "ahead" of the power amps) the main benefit that can potentially result in most circumstances is that each amp channel will only have to supply current and power in part of the frequency range. Which eases the burden on the amp, and can potentially improve its sonics. Also, in the case of vertical biamping (i.e., assigning one stereo amplifier to each speaker) the sonic effects of inter-channel crosstalk that may occur within the amp, if any, are presumably eliminated since both channels of the amp are provided with the same signal.

In the case of active biamping (with an electronic crossover "ahead" of the power amps) each amp channel will only have to supply current, power, and voltage in part of the frequency range. Which can provide further sonic benefits, increase flexibility in choosing amplifiers, and make it possible (although not necessarily practical) to eliminate the speaker’s internal crossover.

Biamping is a more complex undertaking than a lot of people realize, however, and can in many circumstances do more sonic harm than good. Especially if non-identical amplifiers are used.

Good luck. Regards,
-- Al

I've not found that biamping works all that well but there are a ton of variables.

Two of them are amplifier quality and speaker quality.

A third is the quality of the crossover- which is certainly one of the more important variables!

Now the amps I play at home can go to 2Hz with full power, and yet do mids and highs quite well (they are tube amps after all). My speakers go to 20Hz so I run the amps full range. I find that quite often a poor electronic crossover means that you can't get the speakers to blend and often you loose detail (since the crossover is an additional block between the source and amplifiers). I've experienced this a number of times (and used some pretty high end crossovers).

So- success is based on extreme care! If the crossover is designed for the specific speaker used I think you will have greater success.