What creates the effortlessness in sound reproduction?

This is all about distortion.

If higher ordered harmonics not part of the music are present, the ear will sense them as brightness and harshness.


If there are room reflections that are occurring at less than about 10mS or so, the ear will sense them as brightness and harshness.

So the amplifier plays a role- as does the rest of the system! The rule of thumb I use is the system doesn't ever sound loud, even when at 100dB. So I do everything I can to prevent the system making higher ordered harmonic distortion, preventing vibration from affecting the components and insuring that the speakers are properly set up in the room. If all this is done correctly, you will find that many speakers can be quite effortless. I find it easier with higher efficiency speakers as they tend to have less thermal compression. ESLs can work nicely too, since they have no thermal compression at all.

"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