Cans get 'philes closer to the music, simple. I have heard some really sick systems that entail wonderful top of the line speakers, multiple amps, conditioners, and even a designed room layout specifically to deal with sound in the most efficient manner possible. Still the sound does not come close to the intimacy or range in dynamics that most subtle nuances and overtones provide through headphone listening..this is why recording engineers and artists opt to monitor their sound downs via a studio monitor..not a loudspeaker system.
That said, there is the feeling and experience of the physical sound waves of bass that flow through and around the listener that cans cannot simply
NOT provide.. It is this impact along with soundstaging and sheer energy of louder sustained volume levels through listening aloud that gets people excited when listening to a decent hifi. In a
really great system, you needn't pump up the volume aimlessly to retrieve low level detail in the sound and in stead, the fullness and richness in dynamics should be presented via a nice front end and accompanying Pre-amp/amp that shows synergy with the IC and material used..making the music more dynamic.
It is this phenomenom through a proper headphone rig that people are able to achieve many more times the amount of sound quality with their hard earned clams VS speaker listening, but comes at a price in terms of total allowable gain. Since the sound and marks in the higher band take longer to reach the ear and travels through more space when listening to speakers, it becomes difficult without the regular use of an
SPL meter to identify safe sustained listening levels through headphones as cans never "sound louder than are", they are usually louder