LM3886 full amp reduces hi freqs
The 1kohm you removed and presumably shorted would have had a low pass 3dB frequency point (with the 100pF capacitor) of about 1.5MHz - way above audio so shorting it out should have no effect, but there's no guarantee that the correct resistor was fitted or that the 100pF isn't something like 10nF. A 10nF would give a 15 kHz cut-off and a 22nF would be about 7 kHz so maybe this info can help you track down the problem a bit more.