What exactly do you mean by "stock" insulation?
2cooltek sells some sound insulation in the form of Sonex acoustical foam panels. These are like the sound absorbing/dispersing panels that people use in anechoic chambers, just not as nice or expensive. Ian, the guy who runs 2cooltek says he got 4dB decrease with the foam in his case...see that
here. I like 2cooltek, they have great customer service.
Anyway, a company called dB engineering also sells some stuff, but I've never bought from them, so I don't know how they work exactly.
linkage. Hope this is helpful.
Michael