In audacity if you have the appropriate plugin you should see "Glame highpass filte" and "Glame Butterworth high pass filters" under "effects". If you don't have those, then it's time to install them.
http://wiki.audacityteam.org/index.php?title=VST_Plug-insIf you happen to be using linux, try "sox filter <infile> <outfile> filter 120", and you can try over numbers besides 120 of course.
Suggestion... take one song, run it through different roll offs... 40, 60, 80, 100, 120, 160, 200 etc. Then burn a CD with all those versions, listen to it in your living room stereo, your iPod, you car, etc. Pick a particular rolloff that sounds appropriate with them all, not just on computer speakers (for instance). Don't ask your wife's opinion... she will just look at your weird and say "I don't hear much difference". Then go back to the original show, do the whole thing with whatever value you chose, and run it through the same test, to make sure you still like it.