Probably the power switch have you checked it ?

I am also working on a tda2003 based amps i found that if you change the 2.2 ohm resistance for a 68 ohm and the 220 for 1k you get a clean sound. There is still some work to be done but really can play clean bass ,may need some type of freq cut off because at max volume it distorts a little when you play the big string. I use it to drive a speaker with amp sim pedal and fx .please post if you know how to fix big string issuse .