Hi Rumpelstilz!
I wholeheartedly agree. The problem you are describing is exactly what happened to me not only last year at CAE, but also two days ago at CPE. To my disappointment, they used an obsolete stereo both times and this highly affected my performance. The Listening was definitely the worst part in my whole exam. That really gets on my nerves, especially if I think that, at home, I used to score at least 80% using a computer; but during the exam I must have scored around 65%. I doubt it is only a coincidence.
I don't know if they will consider your letter of complaint, and yet I strongly suggest you write it. There is nothing to lose after all.
Last but not least, don't worry about your general exam result. If you did fine in the other Papers, you will definitely pass it. As I told you, we are in the same boat. Last year I had the same problem and my Listening was evaluated almost borderline. Even so, thanks to the other Papers I managed to get a B. So, just one piece of advice: don't lose heart!
Take care.