Problem with audio volume

[CidCammi]

I have a kenwood deck and Hertz ESK 165's. I've broken them in (I think), but as soon as I turn them up to a certain volume they automatically get quieter, and if I manually turn them up after that they distort.. is this a matter of power deficiency or a wiring problem?
thank you

 

[emperorjj1]

what model kenwood is it? what lvl is the deck when they get quieter? once they get quieter what do you do to make them distort? and this happens every time or only sometimes?