A guitarist/singer/songwriter I work with sometimes has a pair of custom "acoustically flat" earplugs. He puts them in and tells me the bass is too loud although his guitar is screaming.
Look at the reduction curves of those plugs some time. Most of them stop drawing the curve at 150Hz or so because below that they are virtually transparent.
That said, I hasten to point out that I have them as well and use them on loud gigs, but with them in my ears the bass is loud in the mix and when I sing all I can hear is my voice in my head.
