This could be a problem with your processor. I had the exact same problem with my old Pre/Pro, I have 2-DIY Maelstrom Sonosubs that I knew were capable of insane levels before mechanical limits were reached, but I kept getting that terrible clack during loud bass passages. I assumed the same as everyone hear, that I was just at the limit of the sub so I turned up the subsonic filter on my Mackie sub amp to about 15Hz this of course helped, but I was still left wanting. Then I tried a new Processor for a free trial period, my old processor was a 5yo Sony TA-E9000ES, a good processor. The new processor was a Parasound Halo C2, I threw in Master and commander and let the auto-cal do all the Levels and Distance, turned off all subsonic filters and set the THX settings to THX ultra sub ON and no limiting or compression of the sub frequencies. The first cannon blast was so startling and incredible loud and clean that I thought fore sure I would launch the driver cones across the room, the shockwave concussions from the cannon blast was so strong you could feel the weight compressing your chest cavity. I had never heard bass from my system so clean and loud. Over the next few weeks, I threw everything I had at it to see if it would bottom out, but nothing can phase these subs now, my Behringer FBD can completely stay in the red for several seconds and still no mechanical limitations can be heard. After some experimenting I have determined that this processor has a much cleaner signal, it's the only reasonable explanation, nothing else has been changed, just the processor. I have not bottomed out my subs once since the switch, every DVD that I had made notes about the bass levels in the past were no problem, plus they are so much louder I can hardly stand to be in the room at reference levels. I’m not disputing what everyone else hear is saying but it’s my experience that the signal going into the amp is directly related to the overall performance of the subs, and before anyone says it, no there was nothing wrong with the old Sony Processor, it was recently back to the factory for a clean bill of health and all the latest firmware and mods were installed, my old Carver Pro had the same problem as well. If you have ever set on a test bench and driven a input signal into a amp stage you can clearly see that when the input signal degrades just a tiny bit the scope will show the output simply amplifies this tiny inaudible trash into a titanic ugly output waveform, after all that is what a amp does, trash in, much louder trash out.