Yeah, I can understand your confusion!! I believe that it exists part because of your admitted lack of understanding of the subject & part because people do not always clearly explain what they mean when they write "easier to drive".
In a nutshell, the issue is the same whether it is a pair of speakers or if it's a pair of headphones. It has to be since it's an amp drive effectively a speaker load.
An amplifier (whether it's a power amp driving a loudspeaker or a head-amp or an inbuilt head-amp inside the iPhone) always delivers power into the (speaker) load. And, power = voltage times current. You an associated voltage swing going to some positive peak, crossing thru zero volts & going to some negative peak (most amplifiers have positive & negative supply rails) & you have associated current flow into (push) the speaker load & out of (pull) the speaker load.
When you have a speaker with an average impedance of, say, 16 Ohms, the amplifier does not need to deliver much current into 16 Ohms to create a fairly large voltage swing because, from Ohms law, voltage = current times resistance. if resistance = 16 Ohms, you need less current compared to when resistance = 8 Ohms to create the same voltage swing. It's the voltage swing that gets the loudspeaker drivers to act in a pistonic fashion to create sound. So, we say that a 16 Ohms speaker is an easy load to the attached power amplifier. Hence the oft written phrase "this is an easy to driver speaker". HOWEVER, there's a flip side: when the current is on the lower side, the voltage is on the higher side!! This happens because power = voltage times current. For comparison purposes, if you are going to feed into a 16 Ohms speaker a fixed amount of power, the amplifier will have to deliver less current (compared to a, say, 4 Ohms speaker) but since the speaker impedance is higher, the corresponding voltage generated is much larger than the, say, 4 Ohms speaker. What can happen is that with very little increase in volume (on your preamp), you can very easily clip the power (or head) amp because the peak-peak voltage swing of the 16 Ohms speaker can escalate very quickly to a large peak-peak voltage swing. Therefore, you often need a power amplifier with a lot of headroom to drive a higher impedance speaker. Power amplifiers with large wattages usually have very large positive & negative power supply rails that can accomodate large peak-peak voltage swings. For example, if you had a 50VAC power transformer secondary, it would generate a ~71VDC supply rail. A 50VAC power transformer secondary into an 8 Ohm load creates a 300W/ch power amplifier capable of accomoating ~70V peak voltages. If you dropped that power transformer to, say, 35VAC then you would have a ~50VDC power supply & a 150W/ch into 8 Ohms power amplifier capable of accomodating only a 50V peak voltage. Normally, a 150W/ch power amp would sufficient to drive most speakers but if you attach a 16 Ohm speaker to a 150W/ch amp & crank up the volume you could easily reach 50Vpk during cresendos in the music thereby clipping the amplifier momentarily. In this case, a 300W/ch power would be better as it could accomodate a 70Vpk voltage swing.
The reverse is true for a loudspeaker that is ,say, 4 Ohms. Here the power amplifier has to deliver 4X more current into 4 Ohms (as compared to a 16 Ohms loudspeaker) to generate the same voltage swing as we tried to generate with the 16 Ohms loudspeaker (remember voltage = current times resistance). So, if the power amplifier does not have a robust power supply, the amplifier will struggle or fail to meet the challenge. Hence, the oft written & mentioned phrase "this speaker is hard to drive". Looking at it from the power delivered angle, since the current drawn from the amplifier is higher (than the 16 Ohms case), the voltage swing is 4X lower (compared to 16 Ohms case) for the same amount of power delivered by the amp to the loudspeaker because if the current goes up 4X, then the voltage has to go down 4X so that the product voltage times current (which is = power) remains constant. In the 4 Ohms loudspeaker case, one can easily do well with the 150W/ch amplifer because the 50VDC power rails will suffice as the voltage swing is less likely to hit 50V during a cresendo BUT the power amplifier needs to have a very robust power supply otherwise the music playback is going to sound very anemic.
All of the above holds exactly true for a headphone - 32 Ohms low impedance headphones require a headamp (internal to iPhone or external) that can deliver lots of current into 32 Ohms so that the headphone boogie when the music plays. This headamp needs to have only a modest power supply rail such as 12V. OTOH, for 600 Ohms higher impedance headphones you need a headamp that has very large voltage supply rails (such as 20V) to accomodate the large peak-peak voltage swings when the music is playing otherwise the headamp will clip. This particular headamp needs to supply only modest amounts of current into 600 Ohms to get the same voltage peak-peak swing as in the 32 Ohms case. In fact, the headamp driving a 600 Ohms headphone needs to provide 600Ohms/32Ohms = 18.75 times less current.
Most of the time 99% of the public is using a headamp that does not have large voltage supply rails to drive a 600 Ohms headphone & the obviously the results are disastrous! A 600 Ohms headphone is not hard to drive - you just have the wrong sort of headamp for the job! The headamp on-hand is very likely the better candidate to drive a 32 Ohm headphone.....
Hope that this explanation helps to clarify. Thanks.
In a nutshell, the issue is the same whether it is a pair of speakers or if it's a pair of headphones. It has to be since it's an amp drive effectively a speaker load.
An amplifier (whether it's a power amp driving a loudspeaker or a head-amp or an inbuilt head-amp inside the iPhone) always delivers power into the (speaker) load. And, power = voltage times current. You an associated voltage swing going to some positive peak, crossing thru zero volts & going to some negative peak (most amplifiers have positive & negative supply rails) & you have associated current flow into (push) the speaker load & out of (pull) the speaker load.
When you have a speaker with an average impedance of, say, 16 Ohms, the amplifier does not need to deliver much current into 16 Ohms to create a fairly large voltage swing because, from Ohms law, voltage = current times resistance. if resistance = 16 Ohms, you need less current compared to when resistance = 8 Ohms to create the same voltage swing. It's the voltage swing that gets the loudspeaker drivers to act in a pistonic fashion to create sound. So, we say that a 16 Ohms speaker is an easy load to the attached power amplifier. Hence the oft written phrase "this is an easy to driver speaker". HOWEVER, there's a flip side: when the current is on the lower side, the voltage is on the higher side!! This happens because power = voltage times current. For comparison purposes, if you are going to feed into a 16 Ohms speaker a fixed amount of power, the amplifier will have to deliver less current (compared to a, say, 4 Ohms speaker) but since the speaker impedance is higher, the corresponding voltage generated is much larger than the, say, 4 Ohms speaker. What can happen is that with very little increase in volume (on your preamp), you can very easily clip the power (or head) amp because the peak-peak voltage swing of the 16 Ohms speaker can escalate very quickly to a large peak-peak voltage swing. Therefore, you often need a power amplifier with a lot of headroom to drive a higher impedance speaker. Power amplifiers with large wattages usually have very large positive & negative power supply rails that can accomodate large peak-peak voltage swings. For example, if you had a 50VAC power transformer secondary, it would generate a ~71VDC supply rail. A 50VAC power transformer secondary into an 8 Ohm load creates a 300W/ch power amplifier capable of accomoating ~70V peak voltages. If you dropped that power transformer to, say, 35VAC then you would have a ~50VDC power supply & a 150W/ch into 8 Ohms power amplifier capable of accomodating only a 50V peak voltage. Normally, a 150W/ch power amp would sufficient to drive most speakers but if you attach a 16 Ohm speaker to a 150W/ch amp & crank up the volume you could easily reach 50Vpk during cresendos in the music thereby clipping the amplifier momentarily. In this case, a 300W/ch power would be better as it could accomodate a 70Vpk voltage swing.
The reverse is true for a loudspeaker that is ,say, 4 Ohms. Here the power amplifier has to deliver 4X more current into 4 Ohms (as compared to a 16 Ohms loudspeaker) to generate the same voltage swing as we tried to generate with the 16 Ohms loudspeaker (remember voltage = current times resistance). So, if the power amplifier does not have a robust power supply, the amplifier will struggle or fail to meet the challenge. Hence, the oft written & mentioned phrase "this speaker is hard to drive". Looking at it from the power delivered angle, since the current drawn from the amplifier is higher (than the 16 Ohms case), the voltage swing is 4X lower (compared to 16 Ohms case) for the same amount of power delivered by the amp to the loudspeaker because if the current goes up 4X, then the voltage has to go down 4X so that the product voltage times current (which is = power) remains constant. In the 4 Ohms loudspeaker case, one can easily do well with the 150W/ch amplifer because the 50VDC power rails will suffice as the voltage swing is less likely to hit 50V during a cresendo BUT the power amplifier needs to have a very robust power supply otherwise the music playback is going to sound very anemic.
All of the above holds exactly true for a headphone - 32 Ohms low impedance headphones require a headamp (internal to iPhone or external) that can deliver lots of current into 32 Ohms so that the headphone boogie when the music plays. This headamp needs to have only a modest power supply rail such as 12V. OTOH, for 600 Ohms higher impedance headphones you need a headamp that has very large voltage supply rails (such as 20V) to accomodate the large peak-peak voltage swings when the music is playing otherwise the headamp will clip. This particular headamp needs to supply only modest amounts of current into 600 Ohms to get the same voltage peak-peak swing as in the 32 Ohms case. In fact, the headamp driving a 600 Ohms headphone needs to provide 600Ohms/32Ohms = 18.75 times less current.
Most of the time 99% of the public is using a headamp that does not have large voltage supply rails to drive a 600 Ohms headphone & the obviously the results are disastrous! A 600 Ohms headphone is not hard to drive - you just have the wrong sort of headamp for the job! The headamp on-hand is very likely the better candidate to drive a 32 Ohm headphone.....
Hope that this explanation helps to clarify. Thanks.