This is one of the best coax for the serious under low volume.
I wanted to do returns for drummer and bass down low in the grave without requiring too large a volume of speaker.
I modeled all Coaxial Beyma HP and other brands. He who gives the best performance in the bass in a small volume (50 to 80 liters).
The 15KX does not happen at the ankle.
Sound quality as very consistent on all coaxial well designed.
The HP millennium are not bad, but at the same power 12KX gives 3 dB more. It is like doubling the power of the amplifier.