Sorry but even if you find who actually made them, OEM factories do NOT talk to the end supplier , your only bet is Radio Shack itself or *maybe* some Authorized Service guy , although obviously he'll prefer you send the dead amp to him.
Of course, if you are experienced and have a well stuffed Lab you can service them anyway without schematic, by applying "general purpose" diagnostic and service procedures, such as checking for presence of correct rail voltages, shorted power and driver transistors, dead PSU diodes , open or missing or unsoldered filter caps, etc.
Living in Argentina I have to do it all day long.
Probably many who live outside USA have a similar experience, working without schematics and lacking access to original parts.
Oh well.