It is possible to put a bit of attenuation in a cable with a resistor. Not entirely sure how to go about measuring your hearing loss and figuring out the correct value, but am reasonably sure it can be done.
It'd be like a balance control, but would be fixed at one value, so you wouldn't need a potentiometer. Once you know the correct value, you could mod the cable with a resistor and then solder it back together with heatshrink covering the resistor and joint. The cable would be about as good as a stock one. Also, it would not be expensive. Maybe $1-$3 for both a good resistor and heatshrink, and 10-15 minutes of work.
Do you know how much attenuation you have on the ear? If you're, for example, -10dB, someone could probably calculate the resistor value for the other side. If you don't know, call your audiologist. They should have a number.
If you don't mind, I'm moving this thread to the DIY Forum. Someone should be able to help you and you might find someone who can make the mod inexpensively.