I’ve heard of some people having success with using the “Bar Snake” (http://www.barsnake.com/), which is similar to filling the bars with other miscellaneous material to deaden the vibrations.
Handlebar height/angle may actually come into play if you experience numbness due to Carpal Tunnel. I had issues last year, and for the most part has gone away after experiementing with different bar risers/angles. As of now I’m using the Rox Risers adjusted so that the bars fall in a pretty natural position for me, and not forcing my wrists to kink and little pressure put on my palms. Now, if I get any numbness it’s normally at the beginning of a ride where i might find myself gripping too tightly.
I use my cruise control like crazy, which gives me a chance to give my hands a quick stretch when I feel them starting to cramp.