MTX Subwoofer Smoking, what could be the cause?

[hispls 5,000+ posts]

If your sub is smoking you are giving it too much power. Either turn it down within it's limits or buy a better sub.

 

[Jeffdachef 5,000+ posts]

I have a kenwood kdc-bt755hd, ill look online and try to find my preout voltage, i believe ive looked before and couldn't find an exactly number.
Link 1: Kenwood - KDC-BT755HD

Link 2: http://manual.kenwood.com/files/IM368_Ref_K_en_02.pdf

Google up how to set gains with a digital multimeter. There is no correct number anyone can tell you, every install and set up is different. Invest in a digital multi meter, you'll need it if you want your sub to have a fighting chance of survival.