Awesome speakers! I spent hours and hours listening to Betas at a shop down in Orlando many years back.
Any chance you can isolate it to just one woofer in the column? It certainly could be a servo related issue, but possibly one or more drivers has a slightly sagging suspension due to age? If that turns out to be the case, you might be able to remove the offending driver & re-mount it after rotating it 180 degrees.
Hope you get everything back to perfect, and enjoy them for many more years!
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