A good trick a live sound guy told me:
if your kick sounds a bit muddy or cardboard, boost the low mids and sweep the frequency until you find the frequency causing this sound. Cut that frequency.
Now you have a little hole in your freq.spectrum which you can fill in the bass channel. (this freqs will usually be between 100-250Hz)
Remember, if you have to EQ something more than +/- 6dB there is something wrong with mic placement, mic choice, the source sound or anything in between.
Hope this helps,
Herwig