Hello and sorry you're having problems. Offhand the intermittent popping sounds point directly to the amplifier IC chips failing. More specifically, failing under normal operating voltages/temperatures after initial startup. To fix this would require at least having the IC's replaced (on average a $150 repair), replacing the plate amplifier which is more cost effective, or replacing the powered subwoofer itself- which you can replace for almost the same cost as repairing it- especially since the unit is 12 years old.
If you need further assistance let me know. Please do not forget to click the 5 STAR RATING, at the top of the page for my efforts. It is my livelihood and you can follow up at any time if needed.