Xen Headphone Set

Product sounds good and has works great to stimulate the Vegas nerve. My only issue is that for some reason after around 4 or 6 months the headphones begin to lose power and sound fidelity. Usually I set the power setting around 10, but then as time goes on I need to up the power and by the 6month mark the power is set at 25 to get the same stimulation. Then when I order a new pair of headphones, they are fresh and begin the cycle all over again with only needing 5-10 power setting. Believe this is a manufacturing durability issue, or perhaps this speaks to what a great product it is that I use it every day and over time it just wears out.