Reasons why a Simple switcher is not a simple rewiring job.
The power supply produces Odd voltages.
5 volts at 10 amp (not an issue)
-31.5 volts for the audio amps
+145 Volts
+15.% Volts.
It has the audio amp build on to the board.
Power for the monitor comes from the the P.S.
You could hack away at your monitor unit and put in your own audio amp.
While it maybe more stable. The resale value of your machine will take a hit.
Maybe you should have someone rebuild your Power supply.