Go to the Markbass website and hit the big red 'panic' button in the upper right corner of the home page. They will get back to you with the nearest authorized service center. Their US nationwide service system seems to finally be totally up and running. Still a few problems, but also some very positive posts.
That being said, assuming that cab has the little piezo tweeter, ANY speaker repair shop could pop another one of those in there for you for very little money. Those piezo tweeter's blowing seem to be a pretty consistent issue with the Markbass cabs that don't have the true compression driver.