why a speaker would sound better with one single a

Biamp rarely works well unless you are very tech-savy and have the right ear and gear to get the job done.

Annalogy: Planes with four engines are less safe than those with two. There are more things to go wrong with four engines than two.

Bignerd100...Don't use the internal X/O. Bypassing it is the main reason to biamp.

Any certified plane operated within its capabilities can lose an engine and be OK. However, while they have enough power to fly on one engine, two-engine planes that lose one on takeoff force the pilot to deal with a sudden 50 percent power loss, and asymetrical thrust, a significant controlability problem. A four engine plane that loses an engine has only a 25 percent power loss.

Which speakers are you talking about?
I've never had this happen.