I see a lot of opinions being thrown around, but is there anyone here who have actually designed, built, and tested loudspeakers and amplifiers? It always amazes me why more people into electronics refuse to learn anything first hand. Do some testing people!
ABSOLUTELY there's a break-in period. I've worked with all of the best manufacturers, Scan-Speak, Seas, SB, you name it. The T/S parameters change DRASTICALLY over time. Get yourselves a DATS from Parts Express:
Dayton Audio DATS Dayton Audio Test System 390-806
It's only $100. And test some drivers before and after break-in. T/S parameters can change from 10-30% and sometimes more! Some Scan-Speak drivers which have been fully broken in can have parameters that are more than 100% different than what the manufacturer publishes
And yes, that directly has an impact on sound. Of course it always sounds better after break-in. The crossovers in any seriously engineered product were tested with fully broken in drivers. What sense would it make to design and test crossovers/loudspeakers with drivers that have not been broken in yet? There would be all sorts of errors.
Now for the amplifiers is a rather different story. ALL electronics perform and sound better after they have fully warmed up. For example, I can connect an amp meter in series with one of the rails feeding the output transistors. As the amplifier is just turned on, each output transistor may draw 80mA of current. After an hour, all transistor start to warm up and the current is now 100mA. After 12 hours, after everything has FULLY warmed up, it may read 115mA. The differences could be drastic. More current per output device almost always equals to lower distortion and better sound.
Offset drift usually stabilizes after the amplifier has fully warmed up.
Resistors are another story. Values change as the amp warms up. All amplifiers are adjusted and fined tuned after reaching max temperature of the circuit. Distortion is significantly higher until this temperature has been reached.