Puff Goes the Sunfire Mk II Subwoofer

Every now and then (and I have not yet discerned a pattern), my sunfire MK II subwoofer makes a soft but very noticeable puff sound. The sound is similar to the thump that you would hear unplugging a heavily amped subwoofer from the wall, but not as loud. My first thought was that I must have a bum cable, but I swapped out the cable and still have the problem. I am running a Sunfire Theater Grand II pre/pro. I also have a nasty ground loop problem which I solved (without any problems until now) with an Entech humbuster box in line between the pre and the sub. The ground effect is coming in from a dirty RF signal from my cable TV provider. I am also using two star-clustered Richard Gray Power Company 400s units which I have all of my stuff plugged into (pre/pro, Sunfire Cinema Grand amp, sub, Kenwood 5900M CD/DVD, Scientific Atlanta Explorer 2100 cable box, Sharpvision 34N-WF5H TV). The problem started when I added the Richard Grays but I don't see how the problem would be related; it seems like a coincidence. Hu-Hu-Hu Help!