I wouldn't describe it as a loophole. I think some of these companies know they have very stiff spiders that will fail in one of a few ways if there's enough input voltage before they've loosened up just a bit, and apply a warranty accordingly. Going easy on drivers for the first couple hours is always good idea. Might save you a torn spider, broken lead, noisy glue joint, etc...We have covered this topic several times including s long debate between myself and aholic in one of them. Break in has nothing to do with keeping the sub from failing/damaging it's self. It will alter the way the plays as it loosens up though.
The main goal for the "break in period" is a loop whole for companies around the warranty.
