Is is possible this thing needs energized to show the short?
Thats right.
My suggestion is to power the secondary side of the Amp with an external power supply behind the Bridge rectifier. So you can take DC.
The failure will occur, but the current limiter of the external power source will avoid further damage.
The main voltage of the amp is not active while this test!
The power for the amp cames only from the external power source.
Check the circuit intensive before make this test, to avoid further damage oft the amp!
LF input must be open for this test - of course
Lock at this Link:
https://www.elektronik-kompendium.de/public/schaerer/endstu_p.htm
At "BILD6" ist shown a typical power supply for A/B amps.
The +12V -12V supplies are the supply for the Power Transistors.
The middle of the transformer secondary coil is connected to GND.
The Value of this voltages are dependent of the power of the amp.
My Idea is to power your amp by an external power supply at the + voltage, the - voltage and GND.
If there is a problem with the power transistors, the current limitation of external power supply will be started.
So you can measure in the Amp without risk of further damage of parts.
It sounds like your amp has an switching power supply instead a normal transformer?
Could it be possible the first transformer is an Power Filter to avoid network distortion?
After the Bridge must be than an big Capacitator.
After this capacitator is an switching power transformer, a power transistor and most time a switching power regulator IC.
At the secondary coil of this transformer is only one diode per Voltage mounted?
What type of power switching IC is used in your amp?
What type from witch manufactor is the amp?
Possibly there could be found a schematic in the internet.
Look at this, that's an typical switching power supply:
**broken link removed**