I have 2008 lt using stock deck with PAC adapter when I turn volume to a certain point bass get quieter

 

Yup that is the stupid built in eq loudness curve. It is in many factory Hu's these days to help prevent damaged speakers. It does it as well when you fade to the rear so if your sub amp is getting it's signal from the rear speakers or sub outs then you will need to leave the fader in the center.

 

Yeah that's what I thought it was sum built in feature to cut distortion and yeah I keep fade and bal centered

 

A few companies like JL audio, Streetwires and Audio Control make interfaces that cure this problem.
Or just change the deck.