S3X-H here.
I had the same problem, I was tracking with my Fender Jazz Bass, and the left speaker sounded distorted, I thought I blew one woofer, so I order a pair of spares, when they arrived, the same thing happened!
As some of you mentioned, it only happens when you solo the bass, and only with this bass, or some kick drums. Weird.
Also, sometimes A LOT of air comes out of the speakers (With Steven Slate Kicks!).
Best regards.
delcosmos.