Have you tried another set of Tubes?I do not think there is a way of blowing the speaker Drivers with a 5 watt tube amp.
Maybe Ian or Alan are better at addressing the problem,but I would change out the tubes to see if that is the problem.
How long is the rated life of the tubes and how much have you been driving the little guy's?
Another thing might be is if you had made any changes to your setup before this started to happen.Often you can trace changes to sound to changes made in the setup itself.Ex. changing the Cables & Interconnects.
Good Luck!
