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have any more specific information as to what you're doing? are you cutting up tracks while recording them? re-mastering audio? or do you want to play with the output from your system?
Sure a decent soundcard (like Asus Xonar or Auzentech) will let you set the LF cutoff. Just don't use a sub and use bookshelves instead. Then for the highs most soundcards cutoff around 20khz anyway. Just use your soundcards EQ to shape the soundcurve with the 16khz+ range reduced.
Although without more information it sounds like you might be using cheap computer speakers with no midrange and you are thinking that by reducing the lows and highs it'll help restore that. Really it'll probably just make it sound muddy though. If this is the case what you really need are a good set of speakers.
M-Audio Av40, Swan d1080, Swan m200mkii, Audioengine A5, Rockr6 (or whatever the heck the thing is called) are all a few good sets of speakers suitable for computer use.