"The worry is not the amount of power that it draws, but is there a heating or any down side effect for leaving the unit turned on full time."

No doubt if you turn it off you will extend life. How much only Axiom can tell you if they've ran reliability models. Thermal stress is the biggest determinant of component life. A 10 deg C rise in temperature halves the mean time between failure of components.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated