Velodyne HGS knock issue on low frequency/high output

I have recently acquired my 2nd HGS 15 subwoofer to serve as a companion to the one I've owned and enjoyed for the past 20 years. I wanted to explore a dual sub configuration, and was able to make a 'barn find' (not literally from a barn) of a lightly used HGS 15 that has been in climate controlled storage for 15+ years. The sub is emaculate - no chips, dings, snags, dents, etc. And it performs very well.....except when certain movie soundtracks or music reach a certain low frequency at earth moving volumes. When a certain frequency and amplitude is encountered, the sub will react with a 'knock' (or series of knocks).

My original sub continues to perform and does not exhibit the same knocking sound of its counterpart sub. The knock sound is not random, is not related to powering on and does not occur continuously. The knock sound occurs only during episodes of low freq/high volume - like an explosion (but not all explosions), very hard thump, etc. The issue can easily be reproduced when playing the same segment of music/soundtrack. The knock is mildly distracting, but generally doesn't occur frequently (depending on the authoring/content).

I've been trying to determine if this is a physical issue with the speaker (spider, surround, etc), with the plate amp, or possibly with the servo. The surround, dust cap and visible parts of the speaker inspect out fine. The speaker doesn't really look its age. But I can't speak for what may be behind the cone. I haven't inspected the amp for swollen capacitors or discolorations. I don't really expect to find anything since the subwoofer, in all other regards, performs perfectly. The cabinet is solid and the speaker and plate amp are all mounted tightly.

To try and determine if the issue is amp, servo or speaker related, and repairable, I have spoken to a local Velodyne repair center, as well Paul at ECB. The local repair center, understandably, won't say what the issue might be without putting it on their bench, and claim that if it is repairable, whether amp or speaker, they can fix it. Paul believes, based on my description, that it is amp related, but didn't offer any further detail or description of why he believes it to be amp related. I'm waiting to hear from him to see if it happens to be servo related, if it can be repaired (short of disabling).

I've done a lot of searching, but have found little that fits my issue from others who have posted Velodyne issues in the past.

I suspect, whatever the issue is, it can be repaired. But I'm not certain. It plays perfectly except for the instance I have described.

But I'm uncertain if the knocking is electronic or physical. I feel comfortable that the subwoofer is in its original condition and has not been tampered with. My guess is that sitting in storage for a lengthy time has resulted in one or more capacitors to dry and fail. If so, then it would make sense to have a professional identify the culprits and replace.

I was hoping someone on this forum could help with offering opinions and experiences with similar issues.

Thanks for taking the time to read through and respond.