Phrank - Sometimes, a different interconnect will have less sibilance. Have you listened to these files on different systems and are certain the sibilance is in the MP3s?

If one must go the EQ route, a parametric is probably preferable to a graphic. This might help explain:

http://www.carstereo.com/help/Articles.cfm?id=53

I'm a bit of a purist and prefer just a preamp and power amp set up. I've heard and used the excellent Z-Systems rdp-1 digital preamplifier that has parametric EQ, and it's transparent as far as I could tell; my issue is futzing with the original recordings. It's expensive, though.

http://www.stereophile.com/solidpreamps/278/

Graphic EQs tend to be cheap and can reduce the quality of your signal, but, as I don't follow this avenue, I cannot provide a truly informed opinion.

Perhaps moving the speakers would help. My guess is that you are getting some strong sound wave reflections off of a hard surface that is adding some harshness to the sound (perhaps around 2,500Hz). Are there windows near the speaker(s)? A hard floor?

I know the sound. I have a wall of glass on the left side of my main listening room that is 32' of glass. At least there are almost no parallel surfaces anywhere.

I have carpeting however (I would trade you the stone!) which helps.

I found that taking advantage of the size of the room helped a lot, especially keeping the speakers a good distance a way from the back wall (five to six feet for me) and far from the side walls (at least ten feet). This way the first reflections off of the glass arrive at the listening position arrives much longer after the direct sound (it takes about 1ms/foot for sound to travel) and is quite a bit lower in level. This mitigates the harshness and mush.

You may want to try some heavy rugs between you and the speakers (at least temporarily) as the sound bouncing up off of a hard floor can be harsh.

Also look at Real Traps' website and consider some sound absorbing panels. I am certain these would help also.

Sibilance occurs in a fairly narrow frequency band centered around 5kHz. It is often prominent in vocal recordings when the singer has been too closely miked. Many studio engineers apply a specialized notch filter called a "de-esser" to reduce amplitude in this band and thus cut back the sibilance. You could do something similar with a good parametric EQ. Experiment with the center frequency, width and depth of the notch until it sounds right.