I must admit that I haven't seen any documentation of speakers degrading with time due to having been used a lot. However, they do develop problems due to long-term exposure to the environment. I'd suggest checking for physical problems, like cracks in the surrounds or in the drivers. Also, the capacitors in their internal crossover networks sometimes degrade with age over the years, and people have reported improvements in sound due to replacing old crossovers with modern, high quality equivalents. Again, it's primarily an aging effect, not due to usage.