I found that the open face helmet doesn't do much to knock down the dB's. This is mostly due to fit I think. A closed face helmet with the "neck gaitor" would do better.

Once you start down the helmet path, you'll likely get distracted by communication systems, particularly if you ride with a partner a fair amount.

I continue to do tweaks to cabin noise dampening. I have recently installed SecondSkin adhesive-backed noise dampening panels on the interior of my roof (where the headliner would be if I had chosen to install that). It really damps the drum head that is the roof panel, it has a nice appearance and "hand", and is very easy to install. It is quite thin / light, which translates to "it will likely not peel off the interior of the roof over time due to gravity and the characteristics of butyl." I hope ... :-)