Having shipped tens of thousands of orders for RAAMmat and PS Ensolite all over the world and helping thousands with exhaust drone issues I have a pretty good idea of what you will need to do.
1) Fix it at the source if it is that loud no deadening can cure it as there is a huge amount of acoustical energy involved. Any exhaust that creates excessive drone is simple not a well engineered, if engineered at all, product. You will need to add a resonator or bigger muffler or whatever is lacking in the system to cure it.
2) To make it better with deadening instead of modifying or replacing the aftermarket system or make a marginally noisy one just right for for some it depends on personal preference and just how loud it is.
Properly placed actual sound deadening mat and foam like Ensolite can help a great deal as well as adding in some egg crate real acoustical foam in qtr panels, under dash, under seats, in the tire well where there are empty spaces, etc...can help as well but they also can absorb moisture so use at your own risk.
How to guide will show you how.
http://www.raamaudio.com/pages/How%252dTo.html
Vehicle fitment guide will give a good idea of materials per area though the qtr panels are smaller than most vehicles.
http://www.raamaudio.com/
Package #1 is the best value and will do all the needed areas and some extras like under the dash to make the whole car a much more pleasant place to spend your time.
Forum members get a $10 discount on Package #1, 1 pack BXT II RAAMmat and 3 yards PS Ensolite, discount code is raampack1
These are world class products, you can pay more for more highly advertised or nicer packages or "specialized" super thick marketing schemes, etc...but you cannot buy better quality
You have my word on that and you can Google up RAAMmat or RAAMaudio
Rick
President and Founder