One thing to keep in mind. Which cushions are on the earbuds greatly affects the sound. The MX580 comes with cushions that are thick but very low in density, which help let the treble through but decreases the bass. If you get thinner more dense earpads, like the ones Radio Shack sells, you will get more bass, however the treble and airiness will be discontinued. To get an idea ibout this effect, while listening to the MX580, press against the backs of the earbuds. You will hear the sound get louder, and the bass to increase, but the treble will decrease.
I don't know where to get extra Sennheiser earpads. I haven't tried to contact Sennheiser. If Sennheiser does sell them directly, they might be expensive.
If you want to try changing the sounda bit, you should try different earpads. They aren't so easy to find(except the Radio Shack ones). I do sometimes swap earpads from other earphones. Unfortunately the dollar store earphones now almost always come without pads. I used to buy them and throw away the earphones just to get the earpads.