Why does bi-amping speakers sound better?

Curious why it sounds better to bi amp speakers vs just running them off one amp?

i am trying to figure out which amp to buy, I am on the fence with bi amp or not.  Speakers are the old infinity kappa 8.1's.  Several years ago when I was married I bi amped my speakers so each speaker was seeing two channels from a parasound hca1500, I think that's the model.... 200w x 2 going to each speaker.  I also tried a single amp powering both speakers so each speaker was seeing 200w x 1.  

is it that I simply doubled the power that resulted in better sound, mostly noticed the low end of the speakers was tighter, more powerful etc.... and obviously I could also play louder.  

Or is is there something about letting one amp not work as hard due to only running high frequencies while the other amp gets to just work on the low end.  

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