Whoever you spoke to gave you bad information. ML has three authorized service centers: Pyramid Audio (TX), United Radio (NY) and Technetron (NYC). All of them have the schematics for your amp.
I would first contact Pyramid, their owner was a tech at the old Levinson brand. As for whether or not it's worth repairing a 25 year old amplifier, a deciding factor is that to get the same performance from a new amplifier, you would most likely have to spend ten times the repair cost.