How about IEMs.

Quote:   Steve Guttenberg, wrote in his cnet blog review thingy, that Fiio E10 stomps D1. Then he continued that E17 sounds even better than E10.

No. Worst idea ever.   The Q701 are OPEN headphones. They leak sound and don't isolate outside noise. It's absolutely pointless to use them in a public space. Not to mention they are damn huge in physical size and they don't fold. Also the cable is 3 meters long.   Buy IEMs or easy to...

OP wanted something small and portable. Nothing beats IEMs in this.

I suggest you read how headphones work. Impedance and sensitivity pretty much determine how much "power" headphones need. If you buy an amp for headphones that don't need it and notice a difference in sound. It is because the amp is coloring it (like EQ). Also it could be because you are...

How about IEMs.

Don't buy portable amplifier. Instead buy IEMs that don't need amp.

It is a known issue. There is no fix to it, afaik.

AKG K550 has high sensitivity and low impedance. It was designed for portable use. It doesn't need an amp.

Yes would be nice if someone could measure the E17...

The HD650 needs something that can output 2.2V into 300-500ohm to reach 110dBSPL. Fiio E17 can output 3V into 300ohm according to specs listed on their website.

The HD25 II is like one of the easiest to drive headphones out there. So you guys are basically saying that every headphone would benefit from an amp. I highly disagree with that statement.

Sennheiser HD-25 II don't need an amp. Use EQ to change the sound characteristics, not an amp.

The K550 is a portable headphone. It's sealed design to isolate you from outside noise. It's easier to drive than HE-400. The cups turn flat for portability. Also K550 simply looks good on your head. The HE-400 are for home use only. Because they lack the above things (they are open headphones...

High gain is used when you can't get enough volume without it.