Gary, it's no wonder that no one will sell you a hearing aid for your 120db ear... Anything that can produce THAT much sound is actually damaging your hearing more, even if that's what it takes to hear in that ear, you are effectively deaf in that ear because any time you are exposed to something you can hear it will further damage the nerves or cilia, even 90db is up there and will cause more hearing damage over time.
Basically what this means is that you have a finite number of hearing experiences left in your life, anything that can reproduce a decent fidelity will also further degrade the hearing organs. You could in theory and in practice produce a device that will give you near perfect hearing... for maybe a few dozen hours worth of time, at which point you'll be totally deaf. You perception of sound has NOTHING to do with the damage that is occurring to the nerves and cilia that are the most easily damaged portions of the human ear.