If you are interested in dialing in a more perfect headphone response, then have a look at the Linkwitz site about reference headphone equalization. It's pure gold.
www.linkwitzlab.com/reference_earphones.htm
Equalizing headphones is like equalizing any other speakers, but most of the correction will be in the treble where headphones get pretty rough. Instead of using an SPL meter, you'll do it by ear. This is surprisingly possible because your hearing is much more sensitive to the treble frequencies than it is to low bass.
I use my PC as a source, and instead of building a hardware network to implement equalization, I use software to do the same thing. I recommend the free media player Foobar2000 with the Convolver DSP. (Windows Media Player will also use the Covolver DSP.) I'll walk you through the equalization process for the software.
1. Turn your headphone amp down. Then use the Room EQ Wizard Generator to listen to sine sweeps. Try a 30 second Log Sweep from 1 kHz to 20 kHz while watching the frequency display. Loop it and listen to it a few times. You should be able to identify frequencies on which treble peaks are centered.
2. Use the Room EQ Wizard EQ Filters to design an EQ curve to attenuate those treble peaks. Your first attempt will be an educated guess.
3. Export the Filters Impulse Response as WAV.
4. Install the Convolver DSP. Open your media player and configure Convolver to reference the Impulse file that you just made.
5. Export a sine sweep from a free tone generator. Listen to it with your media player. Do your filters have the right gain and width? Do you still hear peaks or do you hear dips where the peaks used to be? The sweep will sound flat when the filters are right. This will take a few iterations of adjusting the filters in REW and listening again through the media player.
[This back and forth between REW and your media player may be eliminated in the future. In a future development of REW you may be able to listen to sweeps through your filters.]
6. Now you can add wide band filters if you want to adjust the tone.
For example, listening to my Sennheiser HD 650 I identified mild peaks near 3 kHz and 5 kHz, and stronger peaks near 8 kHz and 14 kHz. I also wanted to tone down the mid-bass a little. My final filters to accomplish this were:
Filter 1: ON PK Fc 3,000.0Hz Gain -4.0dB Q 3.5
Filter 2: ON PK Fc 5,250.0Hz Gain -3.0dB Q 7.0
Filter 3: ON PK Fc 7,750.0Hz Gain -5.0dB Q 7.0
Filter 4: ON PK Fc 14,000.0Hz Gain -6.0dB Q 7.0
Filter 5: ON PK Fc 120.0Hz Gain -3.0dB Q 0.500
8. Headphone paradise. It sounds so good I can't believe it. Convolver works flawlessly.
Feel free to ask me for help if you get stuck!