The mod is quite simple, but a little fiddly as there is a SM resistor to move. You need to rotate R42, then join the open end of the resistor to the return socket via a wire. The FX level pot will now act as an overall vol pot. Before: After:

How to modify or create a DIY-dual switch with LED and 9 V input, to select Drive ON/OFF and Reverb ON/OFF, making the LED be On when the effect (Drive/Reverb) is On, and not the other way around? This is something I modified for the remote for my Laney LC-30 ii Guitar Tube Amplifier, but can also apply to other amplifiers that have the same behaviour when using a remote control with two switches and two LEDs, where one of the buttons is working inverted. Normaly the Reverb is On when the switch is off, and the Drive is On when the switch is On. When the switch gets it's power from the amp, LED will lit when the switch is Off, making the Drive LED be inverse (ON=Off).  So, a small change in the circuit, will make it more logical, Light ON = Effect ON!