Ok now I get it.... Everything you said so far turned out to be true. There is no doubt that my IEM sound better after sending it back for some reason but that's not what made the big difference.
I run some scenarios and compared the sound from my DAC and here's what I found :
It's looks like the JH16's are very dependent on the source impedance which has a huge effect on how they sound.
Current setup iMac 27 -> opt out -> Yulong D100 MKII -> ( cheap earbuds - Shure SE535 - JH16 )
My DAC has 2 headphone output.. Low and high... I fired a Stereophile Test CD and compared the sound output directly from iMac, from the low headphone output and the high one on the 3 phones.
With the cheap earbuds, there was no difference to note between the iMac analog output and low output of my DAC. Plugging it in the High impedance output, the sound seemed louder for obvious reason but not different ( Dynamic driver )
With the SE535, the was a noticeable improvement in sound quality from the DAC ( low Z output ) compared to the iMac output. When I connected it to the high output socket and reduced the volume to match the same dB level of Low output, the bass around 30-60Hz was punchier and deeper with wider sound stage.
Now with the JH16 is a different story. Directly from the iMac, sound was OK but the bass is still uncontrolled, it's boomy to a certain extent which is the case when connected to the iphone 4S and Lumia 920. The bass is sometimes disturbing unsuitable for jazz, blues, classical and opera music but good for techno, hip hop and DnB genres.
It's when I connected the JH16's to the low impedance output of my DAC and played track 12 "Bruce Dunlap-'Threedledum' that took me to an entire new sonic level. Much better clarity, very controlled yet powerful bass even at very high volume level.
Playing the same track from the High Z output, I almost missed half the details...Bass was covering most of the mids even at low volumes. It seemed like if the amp was powering the bass drivers only.
What is surprising is that my Shure Se535 with 36 ohms impedance wasn't affected by the source impedance as much as the 18 ohm JH16.
I've always thought that the IEM impedance isn't a major player since most of them are high efficient within 16- 40 ohms and the frequency response deviation shouldn't be more than 2dB which is not audible and any damping factor over 10 is said to have negligible effect on the sound.