Flat response is always preferred; otherwise you really won't know what you're listening to and won't be able to accurately determine if your mix is too bassy, thin, etc. If you simply cannot invest in near-field monitors, get the best quality headphones you can afford.

I did some quick research for you and found this page:

http://gilmore2.chem.northwestern.edu/articles/hguide_art.htm

Scroll down to the section about frequency response near the bottom.

It seems that, because of the proximity of the headphone speaker to our ear, and our ear's naturally unflat frequency response, headphones need to be eq'ed to attempt to represent the way we'd hear the source if it were right in front of us. I'm no expert, but check out the article. It seems to me that you want a decent quality diffuse field eq'ed set.

Also, when you're headphone shopping, have a listen to different pairs if you can. Bring along a CD of mixes you've done or heard on monitor speakers and see which set translates best.

Good luck! And save for some affordable nearfield monitors! They're probably not that much more expensive than quality headphones (although i have no idea what the prices are like in Brazil)

I recommend listening to as many pairs as possible and make notes. Then choose the ones you find have the flattest response. I chose the DT 880 pro.

Save your money for decent near-fields and room treatment to go with them. Total budget = $3000 to $4000K.

no headphones have a flat frequency response. they would sound funny, because of the proximity to the ears. whatever you buy you will need to learn the sound of, which is a time investment but pays back. the one very important thing i think you'll lose is the sense of dynamics - headphones seem to compress sound quite a bit so monitor speakers are a must.

I am also using just headphones until I can actually afford to spruce up my studio with decent monitors. I would suggest that you don't consider a mix complete until you've listened to it on a few other systems. For example, make a CD and play it on your home or car stereo system or ipod. Basically, play it on something you expect your average consumer to use. This way you could appreciate the difference between what you're hearing on your headphones and what you hear on other audio systems.

That's a sticky situation. People are definitely correct about headphones not being able to reproduce a flat frequency spectrum. The other issue that you're going to have is that your spatial perception of the sound field will also be VERY different than if you were using a pair of monitors.

Personally, I'd rather mix on a pair of speakers, even if they're just a cheap pair of M-Audio speakers.

I wouldn't suggest mixing with headphones in any case...unless u really know them inside out and know what to compensate for...which is something which takes time...and there r some good near fields that could substitute your need of a costly pair of headphones...KRK makes some amazing low budget but great near fields...

I have mixed on headsets for years. Make sure you are using as close to a flat as you can get with no compression or filters. I also reference my room monitors so you need both. Headsets are great for aliening stereo and over all balance of instruments. Be careful with effects especially verb. You may have a tendency to over effect on headsets. The bottom line is whether you have a room monitor or headsets is adjusting to your reference. Check what is missing in your reference by using everything from car stereo to home monitors. I have mastered on headsets by knowing my reference. An engineer without the use of headsets is a doctor without a stethoscope. I have been professional engineer for 35 years. I haven't AB-ed headsets since the 2006 Namn show but find Beyerdynamic and Sennheiser to be front of the field for headsets. I have resently reviewed the Beyerdynamic DT 880 Pro, Audio-technica ATH-M50s, and the Sennheiser HD 380 Pro. I find the DT 880 Pro hands down better headsets for mixing. Sennheiser HD700 and Sennheiser HD600 are Sennheiser's high quality. I have not heard these but I assume they are close to the DT 880 Pro. One last comment. I feel the open backed headsets for professional mixing are the best. Because the sound escapes it eliminates the natural compression that occurs on tracking closed headsets where you are combating external sound. It simulates more what you might hear from your room monitors. Hope this information helps as we are all still learning. Happy mixing!

If you're gonna spend a good amount of money on some headphones for a flat response, which still wont be flat, you might as well spend the money on a decent pair of monitors. Not only will it help you in the long run for your mixes but the less you'll be getting ear fatigue while trying to mix on a pair of headphones. I don't know what kind of work your trying to do and it might help for a bit to work on headphones because its cheaper, it'll be worth the investment in the long run to buy some monitors, for the sake of your hearing, your mixes and overall work ethic.

totally agree with fred renick. U really need to double check with earphones to make sure where in the stereofield your instruments are specially if you use lots of synths that need to be competing for the same range of frequency to avoid masking. In other words, make everything fit in a 3d space sound. No matter how flat ur speakers are, you wont get that picture in any other way.