I ruled out the amp when the OP ID'd his cab as a 2X12 and only one of the two speakers is exhibiting the issue. In doing this, however, I assumed the speakers were wired in parallel (or possibly, but less likely, series) with one speaker cord from the amp to the cab. Though unlikely, perhaps I should have asked first whether in fact, as I used to do with my 2X12 (before I decided I never wanted to cart a JBL-loaded 2X12 ever again in my life, sold it, and replaced it with two 1X12s), the OP is driving each speaker independently.
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