At least you've given new life to most of the board by changing parts
I think you're at the point where you'll need a Cro to analyse the audio signal and follow it around the machine to see where it starts/stops, and look at all supplies carefully...even though you're replacing caps, maybe their too low in value by design?
for example, a large 470uf or 1,000uf might actually need an extra 1,000 to 4,700uf wired across it, to assist filtering....You might find the hum go instantly !
There might be a LAH service bulletin that states "filter cap too low in supply xxx, change to 10,000uf"
who knows ???
Maybe mark's (quiet) machine has this different cap fitted ????????
Other than that, the easy solution as a tempo measure will be the amplifier-gain mod, altering some resistors to quieten it.
Pity you dont have another LAH you could swap cards across with and see if the hum moves to the other machine.......not that i think mark will let you do that hehehehehe.
MM.
