For reference:
my Challengers underside is undercoated (rubberized) and the interior is lined (except for the glass) with a material of high density foam sandwiched between reflective metal liners (cabin insulation for temps and some sound) doubled up in some places.
the last couple thousand miles on longer trips Ive had to put an ear plug loosely in my left ear, else for the next day and a half, its like having a tincan over your ear. Thats been my experience so far.
After I get done with the new heads and Six Pack install, if the exhaust note is still droning, Im going to switch mufflers to something else.
YMMV.
Robb