What creates the effortlessness in sound reproduction?

"The rule of thumb I use is the system doesn't ever sound loud, even when at 100dB..."

Exactly my experience.

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