Newer Iphones after the Iphone 4 sound muffled over Headphones

Hi to all,

I observe a strange effect that is so obvious, that it must have been shown in any

review of any Iphone after the Iphone 4, the effect is bad frequency response:

I am using Koss Porta Pro Headphones but the transducers are soldered to the original Earbuds`

cable to adjust volume and use the mic. The Koss´ average Impedance of 60ohm is a no-brainer.

Sounds great on the Iphone 4 but sounds muffled and dull on a 4s as well as on a 6s.

Of course, EQ is set to flat on all devices and I did not only use Itunes for comparison but also

File Explorer, VLC and Neutron. Neutron and VLC even claim to run

independed from the Iphones´ sound settings (But do not use their own audio drivers)

But the effect is always the same:

Crisp and clear on a 4, muffled on a 4s and 6s.

Of course I used 16bit, uncompressed music, so no mp3 or aac.

You need a little bit of trained ears (I am a sound engineer)

but the effect is obvious. If this loss of treble would have been displayed on a frequency graph,

it would have bee more than 3db, beginning in the 5kHz range - this is a lot, as the db scale is logarithmic.

But all measurements published (see Ken Rockwell or Soomal for the 6s) are ruler flat.

If this loss of treble would have shown in reviews, we would have an

"Audiogate", after "Antennagate", "Bentgate" etc.

So, tests show no problems but testers do not use earbuds for frequency response testing,

the plug in their own cables. No fancy electronics in the signal path to the audio analyzer.

And they do not use earbuds for listening testes for sure, earbuds are too inferior.

This is why I soldered Koss headphones to the cable.

So, here comes the question:

Could it be, beginning with the Iphone 4s, that Iphones recognise

what Apple device/earbud is plugged in the 3.5mm headphone jack - and compensate frequency response?

Earbuds are rather shrill, being "in ears" with very small transducers and not over ears,

where the transducer can be biger for even frequency response.

Thus it could be logical that any device after the iphone 4 defeats / enhances the EQ

according to the frequency response of the earbud - without any notice to the user.

Why?

Because in Neutron I have a strange effect: When inserting the plug while music is playing,

(EQ in Neutron and Music is OFF) the sound seems brighter for a tad of a second after insertion is completed - and then becomes muffled.

Could anyone shed a light on this?

Thanks!