any decent drum sound comes from experimentation. great drums can sometimes tune themselves (sound great even if out of tune..it's rare, but it happens), but most drums are usually improperly tuned.
start with new heads. i'm assuming you are using this in a jazz setting, because typically 18's aren't used for much else...try a Fibreskyn with Powerstroke on the batter side and front, or Powerstroke coateds on the front/batter. the powerstroke reduces (or eliminates) the need for any internal muffling like a pillow or EQ Pad or even a felt strip. from there, tune both heads equally, get them singing the same note. start low (1/2 turn past the wobble stage) and play the drum. if you don't like, adjust up equally on both heads a little. there's an optimal tuning range for every drum, and you'll know when you find what's right for your 18". you should be able to hear that sweet spot. if you still have a hard time, try some different heads (if the drum sounds thin, go for a thicker head or vice versa if it sounds dead..) if there's too much overtone, use a felt on the front head and experiment with that.
personally I enjoy experimenting with tunings...i sometimes use my 12" rack tom as a floor tuned way down
