The Mojotone Mosfet switching board is a highly configurable switching system for your amp. The GA5 mod schematic show an example of how this board can be used for a fairly sophisticated modification. With the included 2n7000 signal mosfets the source is grounded, the drain is run to a point in your circuit where grounding control is useful and the gate is run to a control line from the switching board (C1-C6). To switch an LED connect a mosfet source to ground, the gate to a control line (C1-C6) and then run a 130 ohm resistor from R1 to an LED and the LED to the drain of the mosfet. See the GA5 mod schematic for an illustration of this.
Documentation: Mosfet Switching Document
, GA5 post modification
, GA5 pre modification
Kit includes:  2n7000 small signal mosfets, Complete mosfet switching assembly, and complete documentation (listed here)