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Test the driver (or sound-producing device that is going to the subwoofer) with some other speaker.
Once you have confirmed where the problem is, we can either troubleshoot the sub, or the amp/driver/etc.
The crackling sounds like a loose cable or slightly 'open' cable or connector.
But the intermittent bass sounds... that could be anything.
Make sure of the connections, wiring, and connectors (none dirty, all undamaged, connectors tight, wires not old/frayed/etc...) first.
Let me know what you think, and what you find out, please. Thanks.