If you have a multi-meter, disconnect the pickup in the guitar, put the meter on the Ohms setting and check the impedance. You can test the each coil and both together. The Ohms value should be around 12k for both coils in series.
Not sure about this amp but if it uses a AC power for the heaters then it will probably need a new transformer unless that output is independently fused. Further if it is AC it would be better to convert it to DC tube heat than to repair to its original setup.
That being said if it uses DC power for the heaters then it could be as simple as replacing some rectifier diodes or again a fuse on the main board.
Marshall makes 3 different models:
http://www.marshallamps.com/product_range....roductRangeId=8
Engl makes 2
http://www.engl-amps.com/index3.html
Randall and VHT
I wonder if they make a special micro dovetail bit to cut those slots.
Too bad there is no data on the site to show the difference from standard nickel or stainless frets. All the demos use a high gain sound so its difficult to know how much the sustain has changed.