DonL
Jack of all trades Master of one
I used to be in the audio repair business and used this technique all the time on both tube and solid state amps. It prevents doing a lot of damage if you didn't find all the blown components.
That said, you have to use a VARIAC® or variable autoformer to do this because this preserves the sine wave of the AC power. Solid state dimmers do not do this. They use SCRs and TRIACs to vary the voltage by delaying the turn on point of each individual cycle of the AC waveform. The result is a non sinusoidal waveform which would not work to power an amplifier or other electronic device.
If it doesn't weigh 10 pounds or more it is not a transformer and won't work.
You can use a variable high power resistor also, If you do not need isolation from the mains using a transformer.
I never have seen a DC transformer that lasted very long. They do not like chopped DC, unless they are designed for it.
I use a light bulb in series with the AC line and it gives a good indication of your situation.
Have Fun.