Back to your original question-I have switched from alloy to carbon bars but I really have not noticed any difference in road buzz. However, I use different sets of wheels and some are much stiffer and translate road vibration much more than others.

I know you have been fitted and it sounds like you have a very neutrral bar to seat height which is good for your shoulder issues. Make sure your saddle is level and not tilted nose down which could throw more weight forward and put it on the arms and shoulders. I had similar issues with should pain from riding but mine was due to the "reach" being too long. Your reach from the saddle to the bars might be something you just double check to make sure most of your weight is on the saddle on your sit bones. Your elbows need to be bent while riding so they are helping to absorb the road vibration. If your arms are straight all the road vibration is being absorbed by your shoulders.