At where I am, the IER-M9 is about four times the price of the MDR-1AM2.
I think the IER-M9 has a fundamentally different sound signature when compared to the MDR-1AM2.
Overall IER-M9 has a very relaxed presentation while the 1AM2 is a very energetic headphone.
Also the 1AM2 is quite unforgiving due to it's metallic like tonality, If your source have jitter or sibilance problems, 1AM2 will show them to you in an instance. IER-M9 on the other hand, makes most sources sound refined/soft.
1AM2 excels when the music is complex, where you are listen to the beat and rhythm of the music. They are not so good for analyzing the microdetails. or plankton sound details. Think Alternative, Rock, Hip Hop or Electronic genre of music is what suits the 1AM2. From what I am hearing, great soundstaging and imaging on the 1AM2 really requires TOTL equipment for these to properly show up. You can really turn up the bass on these without worrying about bass distortion or diaphragm breakup that you get with other headphones when you push the EQ too much. They are surprisingly very easy to drive, more easier than the IER-M9(I will explain this part later on).
IER-M9 on the other hand is more tuned for appreciating the sound stage / imaging of the music, think live performances or unplugged music where you are absorbing the entire musical presentation. IER-M9 doesn't allow any part of the sound to overwhelm. The IER-M9 although is just an iem, I would think that these pair of iem requires quite alot more power than a typical iem/portable cans. I felt that somehow Sony has made the IER-M9 to have a "VTEC" mode, where you really need to turn up the volume on these in order for the last bit of the fast transient response to show up, and because you have to drive them at higher volumes, it means that your source amplification need to have plenty of capacitance/power output in order for the IER-M9 to sound punchy/bitey otherwise, the IER-M9 will behave pretty relaxed(or boring for some).