i've heard a few times that with like Type-X's you should let them warm up for like 2 weeks before you turn them up loud... does that go for all subs or just those?
break in's crap Install it right and listen as you please. Things break in on their own.
Break in is a marketing term almost exclusively reserved for high end equipment. IMO it's a safeguard for the seller in case you drop a load of money on some new speakers and you're not pleased with the sound. They can fall back on the old "You've got to let them break in for two weeks before you can 'realize' their performance."
That's code for - "Tough luck, deal with it until you get used to it and they start to sound normal."
im sorry that is called break in. there is nothing in a woofer that requires warming up to operate at its best conditions. electronics equipment may need a short warm up period but not a woofer. like optidriven says it is hard to discern between an actual break in of the woofer or your own familiarization of the sound that is produced.