I didn't read the whole thread and this isn't the only one on this problem. But, my solution was to remove the subwoofer to get to the controls. The controls are on the far side* where only very small people can go (about 6" high). There were two: gain (volume) and frequency). Turn down the gain and you'll never hear anything, so I turned it about half way down. Turn down the frequency and that helps too—what frequency it affects, I don't know. Both controls were all the way up. Is lower more low frequency? Since the subwoofer is always on, I wired a switch with a light into it and mounted it under the dinette seat. Light on, subwoofer on. I've never turned it on since I did this.
After all, who needs a subwoofer in a travel trailer? If I were a teenager, I guess I would, but I have not been so afflicted in about 50 years.
*Airstream uses at least two brands of subwoofer; some may have the controls accessible from under the table. Count yourself lucky if you do. I am not so blessed.