Walstib, is the crackle volume dependent? How much volume does it take to get the crackle started?
Erik is right. Most likely it is the woofers failing. The surrounds and spiders fail and in some instances the voice coil lacquer deteriorates and the coil becomes unsprung. If you push on the woofer you can feel it grind. Amplifiers do not fail like this. If you can't get to the woofers just take the amp to a local store and plug it into other speakers. If it is working fine then you know for a fact it is the speakers.
Erik is right. Most likely it is the woofers failing. The surrounds and spiders fail and in some instances the voice coil lacquer deteriorates and the coil becomes unsprung. If you push on the woofer you can feel it grind. Amplifiers do not fail like this. If you can't get to the woofers just take the amp to a local store and plug it into other speakers. If it is working fine then you know for a fact it is the speakers.