What creates the effortlessness in sound reproduction?

I agree that it results from all components giving you the desired volume while functioning well within their operational envelope.

@douglas_schroeder : you mentioned that the size matters and big gets better in the end. I also read somewhere saying that big drivers tend to be slower due to their size and mass than the small one? How true is it?

@atmasphere : you mentioned:  "I use is the system doesn't ever sound loud, even when at 100dB". I'm sorry for my ignorance but I am not sure I understand this correctly. Do you mean that in an experiment of several systems which are playing at 100dB, some system will be louder than the rest? 
 

@erik_squires : I checked Gik Acoustics, should I go with bass trap or diffusion panels, how to decide which one I need?  I read some articles online but they are pretty 'technical' for me.

Could you give me a hint or direction? I am living in carpeted, small room 16ft x 16 ft with a pair of Devore Nines. 
Thank you
Huy

Hi Huy,
Talk to them. They'll help you get set up. Have a budget in mind. :)

They are really pros at this.  I'm an amateur by comparison.

Best,
E

As geoffkait indicated, but, in two words, dynamic range. Every listener, should have some experience, to listen to live, unamplified music, to get an understanding of what dynamic range, really is. ( real musicians, in a real space ). I am speaking of music and sound, as dynamic range is also associated with  vision ( photography.and video ).