Why is my Velodyne DD-18 "blowing up? "

Hello--I have my DD-18 connected to an Integra 80.1 preamp with 7 other B&W speakers, 2 monos, 1 5 channel amp, with a 20' long RCA terminated mid-range cable.

Over the past 3 months, after playing music for about an hour at about 75 to 80dcb, all of a sudden, I get a very loud explosive 1 second crackling sound that is coming directly from the Sub? Nothing happens with the other speakers, and so far, the Sub just keeps working fine even after the "explosion?"

it kind of reminds me of my old Meridian DSP 6000 speakers that would Go into clipping mode after 1/2 hour of loud listening and get very hot. The difference is that I had to immediately turn off the Meridian system & let it cool down before they would play music again.

Anyone think they may know what is happening?

Thx in advance
If the sound is a short crack,like striking some metallic, or other material with a hammer,It could be the woofer bottoming out.What I'm saying is,the voice coil former (where the voice wire is wound),might be hitting metal against metal.If this is whats happening,the end of the former may get bent and scape,or break loose etc.Turning down the volume may solve this problem.It could be the amp running out of power too.I don't know if this would cause what you describe.The voice coil bottoming,of traveling to far sounds similar on all subs.If you changed to a lower frequency setting,that could be causing the sub to work harder(cone traveling further) than before.Check your settings to see if the sub is set higher in level than before,or a frequency setting may have been changed.If all the settings are the same,I don't want to say for sure,that the sub is going bad.If is under warranty,maybe it should be checked out.
The after one hour of listening is one thing that has me concerned.Otherwise,I think lowering the level some may solve it,and help stop possibility of damage to the woofer.Changing the EQ settings also,possibly less of the lowest frequency level like 20hz,to 30 hz.
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If turning down the lever does not work I would take it in to a tech asap, chances are it will be an easy fix now but it may end up being something major if you continue to use it. Plus, it is hard to enjoy listening to your rig when you are worrying about equipment that is not working properly.