ground problem?

I have 2 4ohm speakers running parallel to a 2 ohm stable amp.   when I turn on my  car I hear a thump.  And when the volume is turned down I hear a hum sound.  I have messed with the gain many times. Its only turn up maybe a quarter of the way.  bass is deep at 5 on my stereo  and the more I turn it up my highs kick in but it seems like I lose bass.  And I mean I can feel that I lose it.  Its a new car and its factory stereo so i'm running high inputs.  also the subs stopped working last week so I figured a wire came off inside the box which was the problem but the wierd thing was that the ground from the box to the speaker is the one that came off.  it looked corroded and the plastic around the clip was black like it was burned.  I replaced it with larger speaker wire.  any suggestions?

I'd recommend checking all your connections.  Make sure there aren't any bare wires that can short together or ground out.  Also be sure the connections are solid.  The thump is a turn on pop that is common with cheaper amplifiers.  Higher end amplifiers will delay the output stage on turn-on to preven that noise from being produced.  Please list the brands and model numbers of all your equipment (subs, amp, and hi/low converter) as well as listing the size and type of enclosure you are using. 

audiobahn abp12t   4 ohm speakers  400 rms each

box is 14-1/2"H x 30-1/2"W x 15"D

audiobahn 8000t amp 2 ohm stable  800 watts rms

You might want to turn off  the loudness on the radio. That will prevent the bass from fading away when you turn up the volume.

Also do you have the remote connected to the ignition?  That might explain the thump when you turn on the car.

Exactly the amp is turning on when  you turn the key. That means that the amp is turning on before the radio and it should be the other way around.

As for the bass if your factory deck doesn't have a loudness button it might have some sort of built in eq to boost the bass when the volume is down. If that is not the case you might want to check your speakers to see if the tinsel lead is hitting the basket when you turn it up( It happened to my brother before). If it's hitting that might explain the burnt terminal and wire.