I have some thoughts on your system and your use of your system.
1. See if you can open the face plate covering the sub's driver and visually inspect the driver for any damage.
2. Since your sub seems to come back to life after some 15 minutes of rest, this could be a case of overheating and clipping. The strange sound that you hear could be an amplified noise of some static or interference noise flowing into the sub. It could also be the sound of the overstressed and overloaded capacitor inside that is trying to empty itself.
3. Subwoofers work on a slightly different principle as compared to other speakers. For one they have their own amplifiers. The AVR sends a 'pre-out' to the sub which it amplifies internally. Just like other amplifiers, if you overload the amplifier from the source, it could trip. If there is no tripping mechanism, it is possible that the amp drives itself to destruction. If you are going to use nearly 50% of your AVR's power, first go to the the speaker set-up and set up the relative sound level of your sub very low. Then use the sub's amplifier pot behind the sub to set your desired output level.
4. Set the relative sound level of all speakers including sub to a max of 75dB using a SPL meter. You should do this using the test tone available in the AVR.
5. When you are doing this for the sub, you have to adjust both the sound level set up in the AVR and the amplifier pot in the sub. As I said before keep the sound level of the sub in the AVR to a very low level.
6. Use a power conditioner that is at least 1.5 times the max load capacity of your sub. It is also possible that your sub is not able to draw the required current from the mains. See if you also ensure that the power to the AVR and the one to the sub are from different switch boards.
After doing all this, run the system through a movie and see what happens. If things work well, your sub is all right.
I have mentioned this before, but listening to movies at insane volumes WILL destroy your ear. You will certainly get what is called 'Tinnitus'. This is a symptom where you hear a high pitched sound all the time. Additionally, your ear drums could become loose and flabby leading to a permanent deafness. Under no circumstances should the sound level of your system cross 80dB at the most. It is safer to keep it between 70 to 75dB. If you are not able to hear things properly, it is worthwhile to spend money on room acoustics.
You can, of course, write off what I say as the ramblings of an old fool, but there is proven scientific evidence of ear destruction because of sustained noise levels.