Understand I have not used these or similar products. But the article says they have an NRR of 31 decibels *WHEN USED PROPERLY*.
Putting muffs on top of that *might* get you over 40, but I have to go back and do the research. While using muffs + plugs will give you better protection than just one of either, they are not exactly additive as one might think. This is due to the physics of how our ears hear sound. The other thing would be that these plugs *enhance* hearing through the device that sits around your neck, so if you were to use these with muffs, technically when they weren't blocking sound, you would hear just as well as normal even with muffs over top of them.
So I guess I should ask for some clarification to your question?
