Here you have it: my very own, non-professional, 1 hour long, home made track.
To get a better tone from your stiff, brand-new speaker, just play the track through your amp at moderate volume,not exceding the max wattage of the speaker(s).
Obviously, you'll have to play it several times to get any significant improvement in tone (around 15 to 20 times depending on how loud you play them). This track is IME much more effective than just playing music.
It includes some low freq sine wave at -6 dbs to get those cones moving, some pink and white noise, some percussion and the odd moment of silence to stop the coil from overheating.
CAUTION: playing the track at loud volumes through little laptop speakers or similar can result in damage. Just used as directed.... basically, if you knacker something, itīs your own fault. OK?
Iīve used it successfully in the past. How you use it, itīs up to you.
My experience is that broken-in speakers have smoother, clearer highs, much broader and more laidback mids whilst the low end tends to get on slightly better with the rest of the frequencies, especially the low mids.
downloadable wav file here:
http://soundcloud.com/5f6-a/break-in-track-for-guitar