For REALLY good soundproofing, a lot more is needed than just Dynamat.

Dynamat is designed to reduce resonance of panels. In our cars, the floorboards under your feet, and the bottom of the shelf after the worst offenders. If nothing else, put Dynamat on these areas, directly to the fiberglass.

The next steps of soundproofing, now that the resonance is reduced, is to block and absorb the sound. Certain foams can be really good at absorbing sound. Unfortunately to get the really low tones, impractically thick foam is needed. So a 1/2 inch of good closed cell foam will do decent at absorbing the higher tones. Finally, blocking the sound is accomplished with loaded mass damper. This is a really dense piece of vinyl. 1lbs per square foot is good. Attaching it on top of the closed cell foam will decouple it from the panel its attached to, further blocking any leftover resonance from the panel. Optionally, a second layer of this (1/2" foam/loaded mass damper) would improve it even more.

Looking at the stock soundproofing, you can tell a similar practice was used, but with lower quality materials than available today. Plus the original stuff is all dried and cracking now.

For best results, Dynamat only needs to have about 50% coverage in the middle of a large panel (however I myself have 100% coverage as well). Then the foam and mass damper need to be formed exactly over all areas and sealed to the surface (contact cement or strong spray adhesive).

I'm even considering removing the stock foam on the back of the carpet, then using a new high density foam and mass damper under the carpets.