This is a contentious issue with a lot of snake oil and mysticism about it. But it's quite simple really - most pro audio or musical instrument speakers have doped fabric or paper or card suspension. The suspension is the cone surround and the voice coil spider. When brand new this suspension is often stiffer than when the driver has been used well into its working excursion range for a few hours.
If you really want to you can break in the suspension using pink noise or a low frequency sine wave but these methods do have some risk of speaker damage if you don't know what you're doing, so I recommend you just play the cab and don't be surprised if your tone changes a bit in those initial hours.
The change in suspension compliance affects the low frequency response but more importantly it changes the midrange tone by improving the damping of cone ripples when they meet the surround, giving a cleaner, clearer, more honest tone.
These changes are not life-changing. They're interesting from an engineering perspective but rather than obsessing over them, play your bass or guitar, have a listen to what happens, then play some more!
Please note that with our 10CR and 12XN drivers, the break-in period purely relates to a tonal change. It's not like old car engines where you have to treat them more gently for the first few thousand miles.
With guitar drivers where the cones are less strong and the suspension often stiffer, this may be the case but it's not something we've tested. But unless you're chugging and downtuning you probably won't have a low frequency overpowering issue on guitar.