Perhaps you can set up the system with the LFM-1, slowly up the system volume until the popping just begins, then stop the source, turn off the LFM-1 and swap it out for the JBL without changing anything about the receiver. If, when you resume playing the source over again in the same place and there is no popping through the JBL sub, I would say that the receiver is not likely giving a clipped output.

Next, without changing anything on the receiver, you might try putting the LFM-1 back in the system and resume playing the same source but vary the volume on the LFM-1 itself. If the popping continues just as often while you lower the volume level on the LFM-1 almost all the way down, although the popping is not as loud, then there might be a problem with the input stage of the LFM-1. If the popping stops as you lower the volume on the LFM-1 and resumes as you raise the volume again while the overall volume of the bass is not that high, then there may be a circuit-related or physical problem with the LFM-1.

There was a post somewhere about some sub (sorry I don’t remember the details) where some of the material inside the enclosure had come loose. It could be that something is near the driver and “hits” when the transducer travel exceeds a certain amount.

I once had a loudspeaker that I thought had developed a problem with a capacitor in the crossover by the way a certain “scratchy” distortion occurred at certain frequencies and loudness levels. I gently turned the speaker on a different side and the behavior of the problem changed. I deduced that the problem was a physical, not electrical, with one of the drivers. I ended up having to remove, clean and reassemble the rear-most portion of one driver’s magnet structure which had become slightly loose.

In another case I received a new sub damaged in shipment. Apparently the box had been dropped from about three feet up and landed nearly flat on one side. Because the box landed nearly flat on a flat surface, there was not much tearing nor was there too much distress to the outer box. It just looked like normal shipment wear and tear on the outer box. But inside the whole heavy magnet structure of the driver had been broken off the ‘basket’ of the driver ruining the driver’s coil in the process. Then the heavy magnet, free of any restraint, had banged around inside the sub’s enclosure for the remainder of the journey to my home, smashing the internal amplifier electronics in the process. What an awful mess. It was difficult to get FedEx to acknowledge their liability because the packaging had so little distress – many weeks before the situation was resolved.

In any case, if you carefully roll the disconnected LFM-1 side-over-side or end-over-end on a soft surface (you don’t want to damage the painted or ‘plexi’ surfaces), do you hear anything loose inside? If you carefully operate the LFM-1 with one or more of the sides facing downward, does the popping occur in exactly the same way as when the sub is right side up? I’m just thinking that if a piece of something got inside the sub through one of the ports and you could easily remove it, that would save the time and hassle of shipping and exchanging subs.

Can’t think of anything else to suggest. If it is an LFM-1 problem, I’m pretty sure the Outlaws will provide excellent customer service. I hope it all works out soon!