Well, you could try a couple things. One, you can reposition the string silencers until you find a "sweet spot" where they work better than any other place. You could also try different string silencers like leeches, etc... and or you could try installing more silencers on the bowstring.

On the other end of the spectrum you could also try using a heavier arrow weight as the arrow will absorb more energy during the release thus resulting in less being imparted into the bow.

Lastly, if you haven't already done so then limbsavers and a good stabilizer should be able to help in the noise and vibration category but not necessarily much with the string twang.