The Jet City JC20H is an EL84 based guitar amplifier which has had some criticism for sounding a bit harsh or brittle. Mine was no exception. The stock EL84s proved to be substantially mismatched and were ditched for some matched JJ Tesla. However the brittleness proved unmoveable despite many changes to the pre-amp section.
The solution was with the power amp section. A 330pf capacitor across the 47K negative feedback (NFB) resistor marked R12 in the service schematic. This removes the unnecessarily high bandwidth of the output stage, and will also increase high frequency stability. Whatever the cause, the brittleness is gone and the amp sounds a whole lot better. Roll off is 1/ 2Pi * R *C , so 330pF and 47K will equal about 10kHz which is ample for a guitar amplifier.
The simulation below shows the frequency response with and without the 330pF capacitor. The peaking at 14kHz is reduced by about 9dB. Scale is 3dB per division vertically and 2Hz to 20kHz.