What creates the effortlessness in sound reproduction?

Hello, 
I have a chance to listen some speakers in the last 10 years. I notice that there are 2 speakers which produced sound 'effortlessly', even at low or high volume levels (I never tried max levels on them since they are not mine). With this effortlessness, listening to music is very relaxing. 

I wonder what makes the effortlessness in these speakers? 

Please shed some light on this. 

Thank you. 
Huy