Diode clipping mods have their place, but they're nothing special. People want what they can't have, so when something isn't obtainable, people idolize it. But I'd rather have extra tube gain stages, not diodes. His mods served a purpose back when the goal was easy, simple, non-destructive "drop in" mods to a Marshall. Change a resistor or cap here or there, add some diodes, move a couple of wires, done. More distortion.
But there are better ways. They just involve more work.