Hi TrueFX,
Did your friend mean the "volume" on the Receiver or the volume on the subwoofer? Do you mean the sub's gain control was at 80%? I assume that's what you meant.
I'd lower the subwoofer's volume control to about 60% of its rotation range--halfway plus a bit. Then set your AV receiver's subwoofer output level (in the AV receiver menu) to about 0 dB or a bit less--or a few dB more if you can't get enough sub output with your AV receiver volume at your usual setting.
You can check the subwoofer playback with some music CDs, preferably some with well-recorded acoustic or electric bass solos. Listen to the deep bass sounds for any mechanical scraping or scratchy sounds. That may be a warped or damaged voice-coil former that banged against the magnet structure when you heard the loud "pop" and that is now scraping against the magnet's pole piece. Some drivers have the former constructed in such a way that "bottoming" the woofer won't damage it, but it will make a nasty sound when it happens. With others, you'd have to replace the woofer if you hear mechanical scratching, buzzing or scraping sounds.
If you don't hear any of those nasty sounds with electric or acoustic bass music, then your woofer isn't damaged.
I hope your sub's woofer isn't damaged. In any case, a woofer exchange shouldn't be too costly.
