What is wrong with my car speakers?

Question

I just installed my amplifier and a subwoofer but the problem is that once it gets to halfway in volume the speakers seem to stop working right, because the voloume dramatically decreases.

Answer 1

check the polarity of your speakers.  You may have one out of phase.

Answer 2

Need information on what sub and amp you have.

Answer 3

what impedance are the sub or subs and the volume  decreases on what radio or amp