I'm new to this forum, and I saw this thread so I thought I would throw in my 2 cents worth.
I developed pretty loud tinnitus about 3 months ago.. Really had me freaked-out. Went to see my aviation doc, and was told to get a good headset.
So, I shopped around and figured I'd splurge and got the best the market could offer. The Bose A20...
Since I purchased it, I have done two short single day trips, and I have noticed my tinnitus was worse at the end of my duty period. Felt like I had been to a rock concert. I am convinced that the active noise cancelling is the issue.
I think the Bose A20 is a Pandora's Box for our hearing and hearing health. They market it as a safe and secure way to protect your hearing because it blocks out unwanted noise, thus allowing you to lower the volume of the radio. I still need to research this, but I think the ANR technology is simply bombarding our ears with a 2nd inverted-phase wave. We don't hear any sound because of the inverted-phase wave, but our ears are now exposed to twice the amount of pressure energy.
Sat on the jumpseat a few times, wore earplugs with a regular headset and didn't notice any issues after those flights.
I'm selling my Bose A20, avoiding any sort of ANR technology and gonna go get a good 'ol pair of Passive Noise Reduction David-Clarks!