[sonitus mirus]

  I am more of am IEM guy and for me, a great soundstage offers a true and realistic representation of "space" between the instruments AS WELL AS "space" between the stage and the audience (or player and listener).
I suppose there can be personal preferences as far as the level of intimacy but on the spectrum of a bad soundstage it would be one where the listening experience sounds closed in and claustrophobic.  I think many true stage monitors fall into this category.
But then you listen to a IE800 and the level of openness is downright frightening.  An engineering marvel in that regard.  Rivals most headphones.

Again, I think this is all just how you interpret the FR of the headphones.   What else is making these differences that you claim to be hearing?  Are we talking crossfeed and delay?  I can understand how signal processing can impact a sense of 3D audio, but this can work with any pair of headphones, and normally the effects work best with better isolation.  I just don't get what most people consider to be soundstage with headphones.  It doesn't seem all that important to me the way most people describe it.

 

[Music Alchemist]

  Again, I think this is all just how you interpret the FR of the headphones.   What else is making these differences that you claim to be hearing?  Are we talking crossfeed and delay?  I can understand how signal processing can impact a sense of 3D audio, but this can work with any pair of headphones, and normally the effects work best with better isolation.  I just don't get what most people consider to be soundstage with headphones.  It doesn't seem all that important to me the way most people describe it.

 
I also don't care much about soundstage in headphones, but one very noticeable thing is when the drivers and ear cups are larger and everything has more space to "move around" in, know what I mean? Everything is more spread out, and you can more easily pinpoint the location of sounds (commonly referred to as imaging), whereas with a headphone that has small drivers and ear cups, and a smaller soundstage, everything is more congested and more like a wall of sound instead of distinct sounds coming from different places. But like I mentioned earlier, the difference between headphones in soundstage isn't that significant, especially compared to speakers and live sound. As for in-ear phones, I dunno, since I don't really use 'em. It does confuse and intrigue me when I hear about certain IEMs having a more three-dimensional soundstage than circumaural headphones.

 

[castleofargh]

  I also don't care much about soundstage in headphones, but one very noticeable thing is when the drivers and ear cups are larger and everything has more space to "move around" in, know what I mean? Everything is more spread out, and you can more easily pinpoint the location of sounds (commonly referred to as imaging), whereas with a headphone that has small drivers and ear cups, and a smaller soundstage, everything is more congested and more like a wall of sound instead of distinct sounds coming from different places. But like I mentioned earlier, the difference between headphones in soundstage isn't that significant, especially compared to speakers and live sound. As for in-ear phones, I dunno, since I don't really use 'em. It does confuse and intrigue me when I hear about certain IEMs having a more three-dimensional soundstage than circumaural headphones.

the large driver, that's the relation you found, but I'm ready to bet that it still end up being FR related. maybe larger diaphragms tend to have less bass roll off, or better treble extension on average? I'm yet to find something without bass that feels like a big room, or something without trebles that has amazing pinpoint accuracy in positioning.
now that's nothing more than the relation I found ^_^. maybe there is more to it? but I wonder how a headphone with real large diaphragm expresses soundstage/headstage if we roll off both ends of the FR with an EQ? I have plenty of IEMs but only a hd650 as headphone so my own possibilities to experiment are very limited.

 
about IEMs being 3D, my own experience would be that indeed sometimes there is a very obvious feeling that the instruments aren't where they usually are. I had tried one IEMs at a meeting that placed a little tambourine at the back of my head when I usually had it at about the same distance on the left but in front of me. and with the 334, listening to stuff moving around, I had a great feeling of 3D and also a slightly different placement compared to what most stuff gave me.
but when I say that I never mean that it is wider or bigger than most fullsize headphones. to me it isn't. it's just that the instruments are inside a bubble instead of on a line, but the bubble is rather small. my jh13(first gen) are compelling example of this. there is a clear sense of more than a line between the ears + some front stuff where voices and drums are(what I sense with all etymotic IEMs for example). it does feel 3D for real. but that 3D bubble is 99% inside my head, a super small bubble. sure I would say that the 334 were maybe bigger than a super cheap grado. but that's about it. when talking about actual "room size", speakers>headphone>IEMs IMO.
on a side note I feel compelled to say that feeling that 3D has nothing to do with being better or more accurate, it's just a little fun.

 

[Music Alchemist]

  the large driver, that's the relation you found, but I'm ready to bet that it still end up being FR related. maybe larger diaphragms tend to have less bass roll off, or better treble extension on average? I'm yet to find something without bass that feels like a big room, or something without trebles that has amazing pinpoint accuracy in positioning.
now that's nothing more than the relation I found ^_^. maybe there is more to it? but I wonder how a headphone with real large diaphragm expresses soundstage/headstage if we roll off both ends of the FR with an EQ? I have plenty of IEMs but only a hd650 as headphone so my own possibilities to experiment are very limited.

 
I've heard a lot of headphones. It definitely depends on the size and shape of the drivers, to an extent, though FR can contribute to or detract from the sense of soundstage as well. For example, the HE1000 and HD 650 have a taller soundstage. You can literally hear stuff physically coming from a greater height than other headphones, because the drivers are taller instead of rounder. And open headphones with larger drivers have a wider soundstage than closed headphones with smaller drivers, at the least. This is reliant upon the physical design in terms of size and shape, not just FR.

 

[TheGiantHogweed]

   
I've heard a lot of headphones. It definitely depends on the size and shape of the drivers, to an extent, though FR can contribute to or detract from the sense of soundstage as well. For example, the HE1000 and HD 650 have a taller soundstage. You can literally hear stuff physically coming from a greater height than other headphones, because the drivers are taller instead of rounder. And open headphones with larger drivers have a wider soundstage than closed headphones with smaller drivers, at the least. This is reliant upon the physical design in terms of size and shape, not just FR.

I don't have much in the way of wide soundstage headphones but I do have the Audio Technica ATH-AD700s. People have said that their soundstage is very good but not as accurate at others. They seem to be really bright at the top end but lacking at the bottom. They have 53mm angled drivers and I have always felt like I am surrounded by the music. Hearing sounds coming from above, behind and all over the place. These headphones also do sound very spacious. I don't know if I am using the right words to describe there sound very open and not at all closed.
 
The other pair I have are the closed AKG K550s. They sound quite spacious but are not really close to my AD700s.
 
Not that I have ever heard IEMs but I somehow don't think you could get that feeling that you are surrounded by the music quite as much as full size headphones with huge drivers. (I could be wrong though!)
 
My AD700s were very cheap for how good they sound. (£87)
I agree what other people say about the sound on the AD700s though and it pretty much matches the descriptions on the first post of this thread.

 

[mindbomb]

There are 3 features that I think are important. Being open back, having angled drivers to minimize outer ear reflections, and having large drivers. Planar magnetics have the largest drivers, and IEMs have no outer ear reflections, but these tend not to work out well for some reason.
 
I think in general, people underestimate the importance of space and soundstage. Without appreciating them, you can't fully understand why people like the sennheiser hd800, akg k812, beyerdynamic t1, etc.

 

[Music Alchemist]

  I don't have much in the way of wide soundstage headphones but I do have the Audio Technica ATH-AD700s. People have said that their soundstage is very good but not as accurate at others. They seem to be really bright at the top end but lacking at the bottom. They have 53mm angled drivers and I have always felt like I am surrounded my the music. Hearing sounds coming from above, behind and all over the place. These headphones also do sound very spacious. I don't know if I am using the right words to describe there sound very open and not at all closed.
 
My AD700s were very cheap for how good they sound. (£87)
I agree what other people say about the sound on the AD700s though and it pretty much matches the descriptions on the first post of this thread.

 
I used to own that one. (Gotta love them pink grills!) A similar headphone for an even lower price is the Philips SHP9500, which I like a bit more.

 

[Pwn3r4Life]

 
I don't have much in the way of wide soundstage headphones but I do have the Audio Technica ATH-AD700s. People have said that their soundstage is very good but not as accurate at others. They seem to be really bright at the top end but lacking at the bottom. They have 53mm angled drivers and I have always felt like I am surrounded my the music. Hearing sounds coming from above, behind and all over the place. These headphones also do sound very spacious. I don't know if I am using the right words to describe there sound very open and not at all closed.
 
My AD700s were very cheap for how good they sound. (£87)
I agree what other people say about the sound on the AD700s though and it pretty much matches the descriptions on the first post of this thread.

Ironically, I own AD700s and don't find the soundstage anything spectacular. I'll test it more heavily tomorrow and with proper amplification. I find them to be very open and airy sounding, but other than that I wouldn't say their soundstage is that great.
 
My Sony MDR-1R have awesome spacing but terrible imaging and no airiness. Both of these headphones have angled drivers. The AD700's are bigger and less angled than the Sony if that matters.

 

[davidsh]

Have a look into darth nut's 007 review: http://www.head-fi.org/t/40313/stax-sr-007-omega-ii-a-review-after-4-years-of-ownership