sealed subwoofer

If you were playing it loudly for a while, could be.
Try a 100W old school light bulb sealed in the box instead, and see.
Or measure the typical rms input voltage and estimate power from the woofer impedance.
 
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With the lack of details from this new member, it's impossible to speculate with any accuracy what is good, or bad.
For instance, is this sub just itself in a box?
Does it have a plate amp inside?... powered?
Are there any other electronics involved?