B&W ASW610 subwoofer burned out twice now

One of my two B&W ASW610 subwoofers last year experienced a circuit board overheat and burned out.  The fuse didn't protect the unit from whatever caused the burn out.  I use a PS Audio power cord into a PS Audio Power Port so I know it's not that.  My dealer ordered in a new circuit board from B&W and replaced it at their store.  All was well until just now today. I upgraded the cable today and when i went to unplug the unit, the circuit board burned again.  The odor was very strong and I know it's not the fuse. The unit is dead.  I have a mind to toss it out in the garbage but for what it's worth, anyone have any ideas as to why this subwoofer would burn out twice and my other ASW610 operates perfectly.  Same system, same room, they're both about the same age, etc.  It is a lemon?  What am I doing wrong?
So every powered sub I know of uses Class D amps and they tend to be more reliable than this.

Wait, the failure occurred at the same time you removed the plug from the wall?
There is a weird condition that happens in switching power supplies, not the amps.

The issue has to do with very short term power interruptions, in the neighborhood of 1-2 cycles of AC power, though by now I would not expect this issue to remain in professional devices. The issue is that the very short interruption causes the normally switched output transistors to turn linear. When they turn linear, they loose their 80-ish % efficiencies and turn 30-ish % efficiencies. In other words, they convert a lot more power to heat and burn themselves out.
I wonder if this is what the OP had?
Thanks folks.  All excellent feedback in particular how class D amps just fail out of the blue.  Makes sense.  It was 5 years old.  Actually I recalled my steps and it didn't occur when I pulled plug out of the wall.  My error.  I had removed the interconnect cable from the back of my surround sound processor and that's when it blew.  Now the sub was in standby mode and the processor was off.  This is when the board burned up.  The sub was still plugged into the wall but it was still a sudden short term signal interruption.   Lesson learned I guess.  I went ahead and purchased a new one yesterday.  I can use it as a passive sub as millercarbon suggested above.
OP - Don't get me wrong. That condition I mentioned is STILL very much a bad, or incomplete design, and it's just supposition.

It could be anything.

But yes, you can replace the amp with a nice Class D with a DSP front end, giving you expanded capabilities. :)
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