Why does it take so many hours to brea in arc preamps and amps?

Sound will open up and become more dynamic also bass definition will improve. Good luck.

They use tubes.

As to how long why not contact ARC?

BTW, they listen to every unit before it leaves the factory - and I think a single person still does all the final listening - yet another guy named Wendell from a high end audio co. in Minnesota - what's in the water over there??

It's generally put down to the time needed to break in the Teflon caps.

 I've had a Ref 2 phono SE for four years now and still only up to 391 hours ... and I'm not sure I can tell you if it's changing as over this long a timeframe lots of other things change as well. Put it this way if you don't like the sound of it at 200 hours (or for that matter at 20) it's not going to sound that different at 600

It took well over 300+ hours for my REGA Isis valve cdp and REGA Osiris integrated amp to settle in .... the actual burn-in process was longer but less pronounced farther out on the time vs performance curve. At 300plus hours, that timeline point was something of a common threshold that apparently is not unusual for many different brands.

Studies by cognitive psychologists show that sensory experience alters perception.

So, is it the tubes, the caps, or the listener?