Thank you for your reply to my questions.
The volume was usually less than midway on the sub, crossover at 60 - 80,
phase at 0. The Yamaha Amp for the sub-woofer was set at 0 when used with Dolby 5.1 soundtracks. I wonder if leaving the amp "On" instead of "Auto" causes over heating.
I have not tested the driver yet but will do.
The STF-2 is the first sub I've owned. I appreciate clean accurate sound quality, but have to admit that I am a novice when it comes to fine tuning audio systems.
Thanks again, I appreciate HSU sub woofers and support.