I wrote this out in another thread, hope it helps-

You just have to train your ears a bit.
Use this method to help find your problem frequencies:
-Identify what your trying to fix, ie sibilance
-Listen to 1 track with the problem, don't switch songs/artists yet. Try to ignore everything but the problem you hear. Try not to use a track with a lot of distortion(Nine Inch Nails/Pantera/Beastie Boys etc), unless that's all you listen to.
- Set the Q(bandwidth) to it's most narrow setting
- raise the gain of the eq band a lot, like 6 or 8db. it should sound terrible
- now slowly sweep the frequency control up and down the general range of the problem. With sibilance, you know it's a higher frequency, so start at like 750hz and slowly sweep up to 15k, then back down. Do this 2 or 3 times.
- As you sweep, listen for which frequency makes your problem the worst. With sibilance it'll be the frequency that most sounds like an ice pick being jammed into your brain
- Leave it on that frequency and lower the gain back to 0db boost. Wait a minute for your ears to adjust to the flat setting. At first it'll sound a little dull after hearing that awful boost.
-Once your ears adjust, cut that freq. by 3db. Listen to see if the sibilance is tamed at all.
- If it's close, try a small sweep around that freq. If you chose 3.25k, sweep between 2 - 4k.
- Now just fine tune it using the gain and q settings. You often will trade off a wider Q with a lower db cut, and vice versa. Try to use the least drastic cut amount that works.
Sibilance is often in a very narrow range, so a very tight/narrow q setting can work. Trying to reduce mud on the other hand will often require a wider q setting.
- Throw in another cd and see if your settings still work. Fine tune with as many well recorded discs as possible.
* Make sure your tuning your system, not remastering the CD!!!
* If you can, always cut instead of boost.