I have a Paradigm Servo 15 sub which had a muddy integration with my Dynaudio Contour 1.8 floorstanders. I elevated the sub onto 8 number 3 Vibrapods, each with a racket ball resting between the base of the sub and each Vibrapod. My flexible suspended wooden floor was also allowing feedback from the sub to the Rega 25 turntable. The racket ball/Vibrapod suspension cleaned up the sound and eliminated the feedback.
The Auralex SubDude does a good job of that and the ASC SubTrap is even better.
How much have you played around with placement? Where is your crossover set? I find that many people set them far too high. I believe that when a sub is properly set up you should never know that it's on, until you turn it off. Where a subwoofer is placed can have a massive effect on it's sound. My favorite placement trick is to hook up the woofer and put it in the sweet spot where you listen. If you have to use a cheap cable to do this go ahead, it's only temporary. Next, put a piece of music on with a bass solo, I find that upright bass in jazz works well. A CD player with an AB repeat is handy for this. Now walk around the room until you find a spot where it sounds the smoothest and most integraded with the music. This is where the sub should go. This is much less painful and quicker than moving the sub all over the room. After you have the proper placement then is the time to start tweaking with feet, etc.
I second the previous suggestion. However, I would also recommend that when you are moving around the room you get on your hands and knees. You are then placing your ears at the height the subwoofer will be playing.