bass problems....power problems?

I have a 1990 Toyota Celica with a JBL BP600.1 running two Infinity Kappa Perfect subs, paralled for a two ohm load.  I also have an Orion Cobalt CS 150.2 amp powring a pair of Infinity Kappa 6.5" component speakers (front)and an old Rockford Fosgate Punch 100z powering a pair of Infinity Kappa 6.5" coaxial  speakers (rear).   I have a 4 gauge lightning audio wire running from the battery, thru a Streetwires 1 farad cap to my sub amp and another LA 4 gauge wire running to a distribution block and then to my front and rear amps.  I have dual 4 gauge ground wires from the battery to the chassis in addition to the factory grounds.  I have the stock battery and alternator.

   Anyways, my problems.  I have had the same main speaker amps and the cap and a single 4 gauge power wire for about two years with no problems.  I was previously running a Boss 400w RMS bridged amp and a single Kappa Perfect 12" sub....again no probs.  About two months ago i installed the JBL BP600.1, a new 4gauge power wire just for it and a second Kappa Perfect sub.  For about the first month or so the bass was just plain pounding.  Hell, it was blowing air out my trunk lid seal.  However, more recently, the bass has become lower and less tight sounding and even worse, sometimes it starts out sounding good and then fades down lower, especially in songs with lots of mids and highs.  I listen to mostly rock and heavy metal so thats most songs.

   So this is driving me nuts.  Do I have something wrong with my 2 month old JBL amp or is my car not providing enough power?  I have 13.4 volts at the battery and 13.35 at the sub amp.  My connections are tight everywhere.  No headlight dimming and no distortion/problems with the main speaker amps.  If it is a power problem what would help the most?  Optima yellow top battery instead of the stock one?  Yellow top in addition to the stock battery ?  High current alternator?    all of the above?  or am i crazy and my amp is broken?

   thanks for your time.....   

This is kind of a hard one with out seeing the car. But what happens after a while is the subs break in and won’t be as tight but normally they start to hit harder. When the sub volume starts to fade dose the voltage still stay @ 13.5 volts? Also what kind of car is this in?

My car is a '90 Toyota Celica.....and as far as the volt meter readings, when i had my meter on the amp my head was also right by the subs so if there was any fade, i didn't notice it at the time.  Mostly, the amp doesn't play as loud as it used to and turning up the gain doesn't help.  I have it about as high as it can cleanly go.  My first thought was that it is a problem with the gain control on the amp because it is incredibly touchy, a slight turn will take it from too low to just right to friggin distorted....and when i say slight turn i mean VERY slight.