DISCLAIMER: not trying to convince anyone .... no argument here .... just explaining MY reasons for my controversial pro-headphone views ....

These are purely explanatory in nature and in no way meant to diminish anyone's contrary position.
Sounds from your monitor speakers blend in the air, responding to each other & having some affect on each other and the stereo sound stage isn't as discrete as in cans .
true ..... however companies like HeadRoom and others have crossfeed circuits that blend a little bit from each ear to the other one in the same way two speakers do in the room.
The HeadRoom ones even attenuate the freqs as if they're having to cross your head to get to the other ear and put the proper time delay in the crossfeed too.
It does a pretty good job of replicating the way that sound from the left speaker still gets around to your right ear and vice versa.
And the freq. range of my HD650's is 10-39,500 hz so that's more than most monitors.
Your other objection is true, of course. There is no room interaction whatsoever going on ........ but you know, sometimes that's a good thing!
As I have said many times ..... with the HeadRoom amps crossfeed on, my HD650s sound
extremely close to what my monitors sound like in the room.
Soundwise, believe it or not, I can forget which is which.
The biggest difference is there is no real physical impact from deep bass.
But there's no lack of deep bass there for your ears to hear what's happening ......
I use both, ...... I have monitors, of course, and I use them constantly. They're also my main critical listening spot when I want to play some vinyl ...... so I use 'em a lot but if I could only have one or the other ...... I don't know ....... my headphone rig is gonna sound EXACTLY like that no matter where I go .... it's a reference that doesn't change and is very accurate and detailed.
I like it for those reasons ...... if I have a dozen instruments all going along in some dense mix I can hear every little volume change and placement change of each and every instrument.
Slight EQ changes show up ...... the only room sounds I hear are what are on the recording ...... I get REALLY detailed info of what the 'verbs are doing .....
I like 'em .......
