Behringer, like a number of other companies, does not release its schematics. They consider them proprietary, plus any OEM who hands you a schematic is also giving you tacit approval to work on the product. That potentially puts them in the liability loop if you inure yourself. Not all companies have that approach, but that is the sort of reasoning behind it. They do have a network of repair centers, so from their perspective you can get the things running. Try calling Line 6 for schematics, or Nady. They won't send them either. The opposite end of the scale is Peavey, who will send any schematic to anyone, plus sell you parts.
What do you mean the high vo0ltage is shorted? If the low voltage works, then the power supply is at least operating. In general, if the high voltage supplies are shorted, it usually shuts the SMPS down. With the wires to the power amps disconnected, does the high voltage appear on the SMPS pins? You may be correct in your diagnosis, but usually, the power amps are what is shorted.
I can't send you the schematic, but it appears that Elektrotanya has the PMP6000 files, and I believe that includes the SMPSU29 board.