There are two mods: the first one makes the percussion 7 dB louder, the second one gives 10 dB boost to the rotating speaker output.
First determine the PCB rev. It's on the PCB silkscreen near the rear panel.
To make the percussion louder for PCB versions V3_1AA through V3_1DD solder in a 620 ohm resistor over R158.
To make the percussion louder for PCB versions V3_1EE through V3_2BB solder in a 620 ohm resistor over R159.
To increase the rotating speaker output for PCB versions V3_1AA through V3_1DD remove R179 (22K ohms) next to C185. Solder in a 68K ohm resistor in its place.
To increase the rotating speaker output for PCB versions V3_1EE through V3_2BB remove R178 (22K ohms) next to C189. Solder in a 68K ohm resistor in its place.
Happy soldering,
—Dave