There are specs like impedance and sensitivity and the output power of your source (PCDP, DAP, smart phone...etc) that you can determine if if needs an amp or not.  There's also common knowledge you can search here to rule it out or not (HD600-->iDevice = Fail).  You can go the cheap route and get a $20-30 CMOY or Fiio amp and see what it does for headphones to determine if amping is needed and/or beneficial at all.

Quote:

Originally Posted by charger35 

What is the best way to determine if you should buy an amp for your headphones?  Is there a certain impedance that headphones should have before you consider an amp?

The simplest observation would be that your headphones are not loud enough, you have to crank the volume to 90%+ to get them to decent levels.

Impedance wise, its not that simple, but usually, beyond a certain point (around 250 Ohms), you'll feel the need for an amp.

I would put that limit more around the 80-100 ohm range personally.  If I run my 70ohm (and pretty high sensitivity) HD25's straight off my iPod, I have to turn it up louder than I am comfortable with to get decent volume out of most tracks, around the 70% mark if I remember correctly.
 

Quote:

Originally Posted by Mutnat 

I would put that limit more around the 80-100 ohm range personally.  If I run my 70ohm (and pretty high sensitivity) HD25's straight off my iPod, I have to turn it up louder than I am comfortable with to get decent volume out of most tracks, around the 70% mark if I remember correctly.

There is no single fixed "you need an amp above this but not below" limit, there are too many factors to take into account. Headphones have a wide range of sensitivity (the 250 Ω T70 needs less voltage than the 62 Ω K701, and even the 600 Ω DT880 needs only half as much voltage as the much lower impedance HE-6), devices have varying output impedance and maximum voltage/current, some musical genres need much higher peak power for the same overall loudness than others, and the normal listening volume is also quite different for each person. All these combined can make a huge difference to what is "enough power".

How much will a cheap amplifier like the Fiio E5 or E11 affect a pair of Sennheiser HD25s?

Is the difference that much to actually make me want to carry these around? It's not really the price I'm worried about, it's the portability factor here.

Quote:

Originally Posted by fountainpenman 

How much will a cheap amplifier like the Fiio E5 or E11 affect a pair of Sennheiser HD25s?

Is the difference that much to actually make me want to carry these around? It's not really the price I'm worried about, it's the portability factor here.

It would be best to try it out and then decide.

Taken from 

Basic Guide To In Ear Canalphones

http://www.head-fi.org/a/basic-guide-to-in-ear-canalphones

The benefit of an amp is usually an increase in clarity. Compared to an iPhone, there will often be a bit of a boost in the bass too. Some people consider those things to improve their listening experience, but it depends on the kind of music you listen to whether or not those things will.

Any amp has different style, so the problem is how can I listen various music via one amp...However I don't want to buy 3 or 4 amps for different music....

I only have experience with an amp that most people up here have never heard of and probably would never use because of some of it's well known "issues". However,....it has a loudness button function that makes all the difference. I am pretty sure most amps kind of have at least some of this function built into them. That being said, even my M50's that most say do not have to be amp'ed, benifit a lot from them. I never use my M50's with out my amp. The amp just makes all the difference. It is not about how loud the volume can get in your cans. It is about how well they sound at both low and high volume. With my cans at arms length and volume adjusted so I can barely hear them, when I put them on, everything, highs and lows are there in abundance. (And yes I know the loudness function is for low volume listening but the improvement is sustained through out the volume levels on this amp) Adjust the arm length sound of your current set up to just being able to hear your cans. Now put them on. Listen to them.  If you find the arms length sound is not full, chances are, you would do well to get an amp.(I mean full, well rounded, good bass and treble, not loud) Even some of the high end source units benifit from an amp from what I have gathered up here. That's my rookie two cents worth, now some of the veterans please chime in and give some suggestions of some good sounding amps.

Oppps just noticed you are talking about IEM's I think, so the arm length test my not apply. Any way, hope I still had some good points for amps in general.