Quote The Elf:
Some day I'd like to get the amp in mine working properly again, which failed shortly after having the headphone jack converted to a line output. The engineer that looked at it was baffled as to the cause.
It's a pretty simple transistor amp, although it retains an old-schol transformer-coupling with feedback to the push-pull output transistors. The power amp stage also includes the reverb amp driver and return mixing circuitry, so the easiest way of deriving a line output would be to tap off the output of the main amp and simply pad it down to an appropriate level.
If you wanted to be able to kill the organ speakers when using the line output that gets more complicated because you'd have to either provide a dummy load in place of the speakers, or modify the amp circuitry to isolate the output transistors to avoid killing them when off load.
Technical Editor, Sound On Sound