Barring physical damage, diaphragm speakers fail due to being overdriven and subjected to sustained DC + or - voltages.
DCV offsets will drive a cone to X-max (one way for positive, one way for negative) and the voice coil will heat up. Quality speakers have shorting rings at X-max that prevent this heating. Small transducers probably don't as they have a << a mm of cone travel. I will be looking at the drive for the 900 series BELs to see if this is a problem. I also suspect that the audio drive may pass DC at high volume levels.
My poll question is have you had a speaker fail in a BEL?
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