Found this on celestions site:
I'll try it out and keep you informed:
How do I break in my speakers?
Important Note! Before breaking it in it's advisable to "warm up" the
speaker gently for a few minutes with low-level playing or background hum.
Break in a speaker with a fat, clean tone: turn up the power amp volume
to full, and control the level with the preamp gain. Use a level that
will be quite loud, but not painful in a normal size room.
Have the bass and mid up full, and the treble at least half. On your
guitar, use the middle pick up position (if your guitar has more than
one pick up) and play for 10-15 minutes using lots of open chords, and
chunky percussive playing. This will get the cone moving, and should
excite all the cone modes and get everything to settle in nicely. The
speaker will continue to mature over the years, but this will get it 95%
of the way to tonal perfection in the shortest time.