Traded for a NEW Ampeg 410 HLF a couple years ago and it's always made some strange popping and cracking sounds when pushed hard. I was convinced it wasn't speakers, but I really never verified that. It wasn't that noticeable in a live gig, but just practicing, it would drive me crazy. Got brave this evening and pulled the grill. I removed a screw from the logo, put a longer crew in the hole, and pulled with vice grips. The grill came off ok, no issues there. I tightened every screw in every speaker, the 4 screws in the tweeter, and the screws holding the ports in place. Nothing was exceptionally loose, bit nothing was all that tight either... I took a new soft bristle paintbrush, swept all the dust off everything, which wasn't bad at all & replaced the grill. Popping & cracking noise is gone. I'm not sure what it was, but my suspicion is a plastic to vinyl rubbing of the grill surround against the tolex, or maybe the ports moving against the baffle. Whatever the cause, it's over and was worth the effort if anyone else has the same problem.