It might be the pre amp or the power amp, so we need to audition the pre amp separately.
So use another amplifier to test the pre amp,
pull the loudspeaker jack out of the amps output socket to avoid hearing the noise via the speaker and then use the send socket to see if that is hum free.
If it isn't then we need to consider two possibilities, firstly a failure of the power supply electrolytic smoothing caps, which I would change out any way due to their age as they really don't tend to last as well as most other components.
But I think it's more likely, due to the crackling you describe that one of the power amp transistors and or its driver transistor has failed.
Sometimes this is caused by the bias components of the amp drifting
Any music shop amp repair tech should be able to fix this no problem as its made out of easily sourced, common parts to almost all amps of this type.
Finding an amp repair tech is all about asking your local PA hire company rather than a music shop what they would do.
A music shop will simply try to make you buy a new combo and may offer you something for your broken one in part ex but I doubt it would
Sorry for your loss, it's a bit like having an old friend die isn't it.