Audio Systems: BASS PROBLEMS, ppi amp, fletcher munson

Question
I HAVE A 91 CHEVY SINGLE CAB WITH A KENWOOD KDC-BT742U HEAD UNIT 2 JL AUDIO W7 13.5 A PPI AMP 2400X2. WHEN I HAVE MY STEREO VOLUME LOW THE BASS IS LOUD BUT WHEN I TURN UP THE RADIO THE BASS STARTS TO FADE WHAT COULD CAUSE THIS PROBLEM

Answer
There are two avenues to explore:

ONE:
Settings.  For example, the EQ area of operation may have a loudness function amongst the several settings.  In this example, as the volume increases the bass boost is decreased gradually. It is typical loudness function according to the Fletcher-Munson hearing curve.  It is defeatable by hitting the reset button on the top right corner of the head unit - which should restore all settings to the factory preset conditions.

TWO:
A malfunction in the system, either head unit, power amps or speaker interface.  Things to check for are overload on amp causing it to drop out - which would be abrupt - when the over current point is reached - and come back on abruptly when the current drops back to a normal level. This cold be caused by a shorted voice coil, short in the speaker line, overheating of the amp (from long term hitting it with heavy bass).  Under rated battery could cause it when voltage drops too low causing amp to drop out.

Check the headlights on the truck if they dim at any time during the playing of bass stuff.  Also, check the lights on the amp for blinking or color change which may indicate overload situations.

This system should, when working properly, provide awesome total performance (which is just not exciting when the subs are not pumping out up to their potential!)