Cool Edit pro will do that. don't remove all the bass, but add more treble, that way it just sounds like a ringtone from a lower quality phone, but still doesn't sound like its ripping the speaker apart.
And if you get Cool Edit, you can increase the mp3 volume so you can actually have loud ringers. its sweet!
If you have a Mac, which is likely, use Garage Band to make the ringtone. Then you can EQ the song to have a little less bass and hype the treble. Also, make sure that you export your mp3 with a higher quality.