The absolute cheapest way I know to solve lacking bass and too much highs is to attenuate the highs. Solder the capacitor onto an rca and plug it into the Tape Out jack. Usually, the tape out is connected before the volume control. Right in the signal path. Value? Depends on the frequency the top end starts getting hot. .47mfd ? to start? This is where Radio Shack excelled.
Too bad the amp sucks to you this way. And now having to resort to putting the band aid on your elbow when your knee is bleeding.