candg,I'm not sure if you use any form of an equalizer with your media player,however,if so....then lower the treble a little bit so as to tame the bright highs with your rs-2's and be sure to encode your mp3's to a higher bitrate such as 320.Good headphones such as those will reveal the sound from bad sources(low end soundcard,lousy engineered cd's etc...).As several others have stated and they need time to burn in.
Grado headphones will always have a bright sound..thats part of their signature. Some of us either love that or detest it. If your not pleased with them,return them and experiment with other brands and models. Be aware though,some brands/models have a higher ohm rating and might sound even worse thru your computer unamped. Look at the technical specs when researching headphones and pay close attention to the ohm rating 32,120,300,600 etc...most of those can sound decent but a headphone amp helps to drive them better. Lot's of folks have offered some great suggestions here regarding better soundcards etc....and Grado just may not be your cup of tea.Give it a week or two ok !