Audio Systems: Amp volume level, external amplifier, amp volume

Question
I just finished installing a dvd player, amp, and sub in my 2004 GMC Canyon. Everything seems fine except I'm having a problem with the radio and amp adjusting together. The volume of the amp can only be turned up and down by adjusting the gain on the amp itself. When I adjust the volume on the radio the amp doesn't go up and down with it. The bass settings don't seem to affect the amp/sub either. The DVD player is a Power Acoustik 6500, I think the amp is a 150 watt MTX and the sub is a 250w Punch. I've tried searching google and came up empty. Thanks for your time.

Answer
This is a problem with the head unit.  Some heads have fixed output and variable output to feed the external amplifier.  Check that to see if there are either two output connections or if the setup allows for fixed or variable audio output.  You want to set it such that the audio feed from the head unit adjusts up and down with the volume control setting, obviously.

Then let me know the make and model of the head unit if you need to come back for more information.

PS: There could be other ways the head is doing it, but you have go to find the variable output.