It is, its very obvious if you for example take the Fostex T20/40/50RP-MK3 headphones. They all have the same specs (except the 40 at 91db), but it is quite obvious that there is a volume difference between them. I can tell this because of my modding with the T20 model.
And besides that, you cant measure the impedance that you get from dampening the shell, driver and all that. The impedance you see listed on specs is just the electrical impedance you get from the driver itself.
Statement about the impedance is wrong. Impedance is purely physical (->electrical)property of the driver, there is no impedance derived from filters etc. Filters/dampeners only affect sensitivity.
As for the fostex ones - I think that you may be right in that particular example and maybe it is not standardised between manufacturers on what do they tell in their specs
But it is not a value that you take into any considerations of the power requirement and it is not the impedance from the specification of headphones (so the value in Ohms). Sensitivity may be a function of acousting impedance out of many factors
It does, more acoustic impedance you have and the higher power you need to get it to the same volume as it was before.
This is seen even when you do simply nozzle filter modding, where for example the sony import foam have a pretty big effect on the overall SPL. This is seen when you measure the FR for it and the SPL value is different.
Ok, this is purely academic, we may be talking about different things 
I agree that acoustic impedance directly affects the power requirements, but its value is normally (or should be) taken into account in the Sensitivity value of a driver/headphone datasheet (probably not directly taken into consideration but indirectly).
The impedance in the datasheet is electrical property of a driver and combined with sensitivity gives you the answer on the calculated power requirements.
The filters you buy have acoustic ohm in their specs but it does not add up to the impedance from specs sheet (it increases acoustic impedance → decreasing sensitivity). It is apples and oranges
Long story short, theory and reality are going their own paths very often
Yes, it affects the power requirements.
and yes, they dont add up like that. but power requirements = specs on paper + acoustic impedance from tuning (which is NOT listed).
Thats how you get transducers with super low impedance and very high sensitivity and yet arent super easy to drive. Or vice versa, looks hard to drive on paper but is actually easy.
M15 cant drive Tantalus or serratus on high gain max volume. Doubt it can drive the he6 
I could not find any official statement of headphone manufacturers how do they measure the Sensitivity, but if it it taken from the driver it is totally useless value to state anywhere. It would be even less useful for the speaker system and would lead to a lot of costly mistakes for the buyers. SPL needs to be measured somewhere, for machinery for example there are standards saying from what direction and what distance you need to do the measurement so to later provide it obligatory in instruction for use (as it is safety related). For headphones I guess you do what you want
On the other hand did you measure the SPL of the theoretically easy to drive, but practically “hard to drive” headphones or they were just not “sounding right” at expected source power?
One extra pain the ass is uneven impedance across the frequency range area making the calculations less trustworthy
Was thinking about the Softears Volume (specs super ez, but not irl) and Tanchjim Tanya (average, but hard to drive).
My phone. I don’t get a point of buying a dongle if it’s not to be used with a phone so yeah, I only test dongles with phone/tablet. Might be different on PC, gotta try it!
But I can tell you right now that on phone, they are lying. I had Zen Pro at 60% low gain, and that’s my easiest to drive IEM!
Tanya is indeed a very strange example and I don’t know how to explain that. Or maybe it gets loud but wrong sounding?
However Volume with 5 ohms is normally outside of operating range for most of amplifiers I know, where the power specs are really off (in direction to low values) what is declared for 16-300 Ohms. And it needs relatively large current in this range
That shouldnt matter at all, since we are using the dac and the amp of the dongle.
Unless its some weird stuff like android stock apps or on apple where everything is bottlenecked by lightning.
It matters since phones USB output has current cap at 100mA and I don’t know really how efficient are those dongles? 40%?
And it draws current like crazy apparently
USB OTG is standardized and it allows to get up to 500mA or even more in power delivery mode, but it looks like phone manufacturers limit that value. 100mA was max for a device connected to USB in “non configured” mode and some devices do not exceed that. iPhones are even worse. I cannot find some more official information on that, but what I noticed in M15 was that with phone I have my volume at 60-70% for MEST as Nymz said, and on PC I had MEST at 12/100. I dont know how it is scaled though
What do you want me to measure?
