Hmm I can't really remember - I had a Polk db10 - and it was in use for ohh I think 4 years. I remember it wasnt because the coil got too hot, but there was rubbing between the voice coil and the magnet - it was clearly evident when it was taken apart. I wouldn't really worry about it, I think this was sort of a fluke from what my friend told me - he installs car stereos for a living.