Bottom Line Up Front... I hear NO difference with vs without amp! Sound is indistinguishably excellent either way.
I just received the Fiio E12 (Mont Blanc) headphone amplifier, which I ordered for use with my iPod Touch 5 (and iPod Classic 7) and my new Grado PS-500 headphones.
The construction of the amp is great, and it thoughtfully includes a carrying case, embossed silicon rubber bands for binding to the iPod, charging cord, and 3.5mm male - to - 3.5mm male connector. I also ordered the Fiio L9 line-out dock, which has the 30-pin (older-generation) Apple connector on one end and a 3.5mm male jack on the other, for connecting to the iPod Line Out (rather than the amplified output for headphones).
It sounds fabulous. Its construction is excellent, with a brushed black metal case, volume control with enough turning resistance to stay put when you set it, and a switch for lower-level inputs (boost by 16 dB) and another switch for boosting bass (increasing boost from 0 dB at 200 Hz to a peak boost of + 5dB at 27.5 Hz, lowest A on the piano).
The bass boost is handy to compensate for a bass fall-off of the Grado PS-500 response that begins at 100 Hz and drops down by 10 dB at 27.5 Hz.
Fiio E12 spectrum w/ bass enhance "on" (green line, upper plot) vs. Grado PS-500 low frequency fall-off
The length of the Fiio E12 just matches that of the iPod Touch 5th Gen with Lightening-to-30 pin adapter (sold by Apple for $29.95 and required to convert to the 30 pin format and to reconstruct an analog line out from the digital Lightening connector output) plus L9 connector.
iPod Touch 5 with white Lightening-to-30 pin, Fiio L9 Line-Out Dock, Fiio E12 amp, and Grado PS-500 headphones.
iPod stacked onto Fiio.
Close-up shows Fiio E12 matching length of iPod + connectors.
In like manner, I could stack the Fiio E12 with my iPod Classic 7th gen (ie. the one with 160 GB of storage).
Fiio E12 with iPod Classic 7th Gen (160 GB) and Fiio L9 connector going directly into iPod.
In short, the Fiio E12 is everything you would expect of what is called Fiio's premiere portable amp.
I was so excited to try it, having spent about $135 for Fiio E12 and Fiio L9 connector. I expected that once I introduced it between my iPod and my Grado PS-500s, the stage, microphone, speakers, miles, and years would drop away, and I would be personally hangin' with Herbert (von Karajan), giving him pointers on his interpretation of Beethoven symphonies with the Berlin Philharmonic.
I used the Chesky Ultimate Demonstration Disc, ripped from a CD with lossless Apple compression (responding to someone telling me that the lossy Apple AAC compression would wash away any subtle improvements from an amplifier). I matched the volume levels and did NOT turn on the bass enhance switch.
I listened to orchestral, vocal, jazz, and drums pieces from that disk. I was most impressed by the last track, which is a trap drum set playing solo. I could even envision just where each drum and cymbal was on the sound stage!
But I could do that without regard as to whether I had the amp in the chain from the iPod to the headphones.
I then tried directly from the CD player playing the disk before ripping. Again no difference. I tried my Sennheiser HD 598 headphones instead. No difference. I tried iPod Touch, iPod classic, and iPhone. No difference.
The music sounded FABULOUS, with or without the Fiio E12 amp. But I would never confuse what I heard with a live performance. I listened repeatedly, hoping to find less "cover," more transparency, more precise positioning of musicians, better transient responses, ANYTHING. Sometimes in my determination, I thought I started to hear an improvement when listening through the amp. But then I would switch back to directly connecting the headphone to the iPod, and that improvement would still be there.
I am convinced that if I were presented with the with- and without-amplifier configurations in a blind test, I would be at about 50% correct, which is the same as random chance.
Is it me? Is it the recognized fact that the iPod (particularly the iPod Touch 5th gen, with its 0.73 ohm output impedance) coupled with the Grado PS-500 headphones and their relatively low input impedance (32 ohms, in the range for which iPod is designed), is already matched well enough and cannot be improved by the amp?
Or is it like the Emperor whose tailor claimed he made him a very special new outfit, one that could only be seen by the cognocenti, and anyone who cannot see it should be shunned as an ignoramus (and then the tailor clad him in... NOTHING! But everyone was afraid not to admire his new clothes).
Well, I can still proudly attach my iPod to my Fiio E12 portable amp with those black rubber bands and walk around in audio stores and shows, imitating an audiophile and impressing the staff (just like I wander around our college campus in my Beats headphones, convinced I am impressing the ladies with my kool rapper image!).