Unfortunately that's an "in between" Sunn.
The early 70's were the classic ones, which everybody knows, so they are everywhere.
Then sales went down, maybe they even closed doors, and probably some new investor tried to start it again in the 80's.
Well made amps , but Musiciand are very fad conscious and probably these did not sell well enough.
Then Fender bought the brand, mainly for the PA line .
Fender Box Powered Mixers were actually rebranded Sunn.
Now that I think of it, I'll check some 80's SRsomething Fender schematics, maybe one of them had the same Power Amp.
Bass and PA both need loud clean amps.
If I find a 300W into 2 ohms one, it may be the same.
If not, get some backup amp to practice, because repairing this one is possible and not expensive, but will take a lot of time and patience.
You will need to grab pencil, rubber and paper and start sketching the actual schematic.
Just draw two parallel lines, one on top, one on bottom, they are the + and - rails, go from right to left, starting with the power transistors which somehow go to either rail, either direct or through some resistor, and go backwards.
Maybe with just the right half is enough, because it's the part that suffers most abuse.
Look at the "old" schematic for inspiration and redo the drawing as many times as you wish to keep it clear and reasonable.
If you don't have some part value, label it with its PCB ID anyway (such as R41, etc.) , seeing where it goes I can estimate a reasonable value.
You can do it, but will take time.
No kids, pets, Wifey or TV in the room, so you concentrate.