Back in my drumming days, I went to an audiologist who made me custom ear
plugs with filters. It sounded the same as without them, only the volume was
just turned down. It can be done, it's just expensive. The filters shouldn't
have a completely flat response because your ear canal is shaped funny.
You'll need filters that match the general response curve of the human
middle and inner ear.

My only problem is that I have abnormally small ears and ear canals, and
after a while, my earplugs start to hurt.