PMC GB 1 Speakers - Running In

The purpose of the run in period for most speakers is to break in the suspension components - often resulting in among other effects as a slightly better & deeper bass (it slightly lowers the Q of the woofer as the suspension loosens up) This is all taken into account when a well designed loudspeaker is manufactured just as an automobile engine is designed to anticipate break in. However a loudspeaker cannot be damaged by pushing it hard (within its design limits) before it is broken in as some automobile engines can. There are no parts rubbing against each other in a loudspeaker if it is functioning properly. Voice coils/formers are aligned to maintain a very small critical distance - not actually rub against the pole piece or magnets in the gap they operate in.
So driving a properly designed loudspeaker hard before it is broken in will only slighly compromise its sound temporarily if at all. Now if you were to recone or refoam a speaker driver that would be different as you would need to wait for the adhesives to cure before use. -Torin

Alex,
IMHO, the joy of audio is the journey you experience with your equipment. I'm usually interested in how the new equipment sounds "cold out of the box". If it sounds tinny and bad, I would then subject it to a break-in period of a long week-end at half volume. Perhaps 50 hours. From then on, I would start listening to it and enjoy all of its improvements over time. But that's just me.
Do let us know how you like you GB1. I heard they are good speakers.
PMC speakers tend to require a longer period of break-in time due to their tweeters.