they arent comparable.
When i sit in front of my speaker system (worth around $3.5k in all) im far beyond the point of listening for details. I listen to a lot of jazz: when i audition speakers i look for a wide soundstage, i listen to the how far the music drifts past my ears and how cleanly the sound is able to cut through the environment. I want to be able to close my eyes and can pick out exactly where each member of said jazz quartet would be sitting. This wouldnt be possible with headphones
I've auditioned $25k speakers and i have auditioned HD800s..there is no comparison what soever;quite simply, the highest end speakers will destroy the highest end headphones. They offer that atmospheric feature that headphones just cannot. If i listen to 'A Kind of Blue' i can imagine Miles in front of me, coming from a specific spot as if he is actually standing there. If i listen to it through headphones, this quite simply is impossible as the sound is directed straight into my ears. However, I am sure you have heard the term 'a great headphone' album or the like. Headphones have an ability to reproduce music at extremely intimate levels. Its a completely different experience, which many (myself included) prefer. Personally, one of the reasons IEMs are so great is because the sound feels like it is coming right from the middle of my skull. I could never get this feeling with speakers.
If you were to spend take a $250 pair of speakers or $250 headphones, the headphones would blow them away. But there are many that would choose speakers.
If you were going to spend $2500 on headphones and $2500 on speakers (with comparably priced amps) i think the speakers would take the cake. But there are many that would choose headphones.
The bottom line? they arent comparable, at all. The only similarity they have is that they both reproduce music. It is not the job of headphones to emulate speakers, or vice versa.