I see a common problem with amplifiers that are older that cause some of them to perform dramatically worse than new. The issue is that the knobs and switches that are associated with the low voltage (preamplifier) side of the amplifier can corrode internally. I have seen this problem with JL Audio subwoofers, M&K subwoofers, Parasound amplifiers and Bryston amplifiers along with other equipment with these types of potentiometers and switches. This can also happen with preamplifers or receivers, but I rarely see it in those products.
The most common culprits are volume or gain knobs and balanced/unbalanced switches on amplifiers.
This corrosion manifests itself in two ways. The most common effect is increased noise from the impedance varying. The other is a dramatic drop in sound output from increased impedance.
This can be eliminated in many cases by rotating the volume knobs or switching any switches until the crackling sound goes away or the sound output increases. I generally do this with a pink noise signal present at modest volume levels so I can hear the results when possible.
This can be avoided by purchasing external amplifiers with no analog volume controls or switches in the low voltage part of the signal path. If you own a product with these knobs or switches I would consider rotating the knobs or flipping the switches several times every few years to keep the corrosion at bay.