A blob might have been overstating things I guess. On at least one live recording of Jeff Buckley, if I use my HD-800s out of the FiiO X5 (or with the HD600 and HD650 back when I owned them, with amps of differing quality) it sounds like I'm at the back of the audience, with everything happening in front of me. Out of the Schiit Valhalla or the more expensive amps or DAPs I have here, it sounds like I'm in the middle of the audience. If you happen across someone with an X5 at least (I haven't tested my iDevices recently with the HD-800s) then I suggest comparing if you can, with suitable music.
Currawong, i owe you somewhat of an apology. I've just realized that some of the amps/dacs you mentioned are "TUBE". So, i can confirm that your observations would be correct; do you notice this phenomena of... "like I'm at the back of the audience, with everything happening in front of me, but out of the Schiit Valhalla or the more expensive amps or DAPs I have here, it sounds like I'm in the middle of the audience. "
While i was writing my replies last night i hadn't slept for literally 24hours and forgot to mention what happens with real analog mixing desks and Tube equipment. During song production, when we insert song mixes through analog compressors and Tube equipment and Classic 80's mixing desks, the stereo-field literally 'opens-up further' and additionally the sound gets slightly more three dimensional, much more so than the original static mix. The 'analogue' or 'Tube' situation is a whole different kettle of fish, so i do formally apologize, but you must admit, again as i said before, it's not the headphone doing this, i'm making a concession solely in relation to using a TUBE Amp/DAC.
For anyone interested in how this is, the simple answer is this, for example in 80's analog mixing desks (does anyone remember the 'tone' of 70's and 80's music?) those mixing desks are full of what one would technically call 'imperfections', anomalies as compared to pure digital systems, but it's exactly those anomalies and 'imperfections' that impart their 'character' to the sonic spectrum of the song/mix; these attributes of 'character' can be measured in the circuit path of a mixing console, which appears to add nonlinear dynamic artifacts to an audio signal such as specific types of harmonics, saturations, phase distortions, dynamic frequency deviations, crosstalk, and noise. And for some weird reason, this all affects the music in a way which is deemed or often described as VERY MUSICAL.
However, i will mention that with using a 'TUBE' Amp/DAC, this phenomena i described which happens 'during' song production would not be the case 'after' that fact anywhere near as much, due to the TUBE DAC/Amp having superior specs in relation to distortion, crosstalk, noise and dynamic frequency deviations, these things would be much less than a typical 80's mixing desk, so this phenomena you mentioned would be purely down to the TUBE adding 'saturation' and 'harmonics' which are indeed responsible for adding a bit more lucid 'presence' and 'definition' to the entire sound, thus explaining what you are hearing.