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

It's the massive teflon capacitors that need the break-in time.  I have a ARC Ref 10 preamp and I know it can be an agonizing process to wait for the full blume and potential, but as others have said its well worth the wait.  I can honestly say, having listened to and owned many other preamps, the ARC Ref 10 is the best I've heard (after 600 - 650 hrs of break-in).  It's within a hair of the VAC Statement that costs over twice as much as the Ref 10.  Hang in there your patinece will be rewarded. 
Ray

Excellent advice- Ray (rspyder).
Happy Listening!

Nelson Pass IS an Authority.  But he is simply stating what numerous experiments by cognitive scientists have shown.

But ARC made their amp and can advise you best -- even if listener subjectivity is an effect, that does not rule out teflon caps needing some time.

Also, as ARC how long they listened to your unit - they listen to every unit they make not just a sampling.

randy-11
Nelson Pass IS an Authority

Exactly! That’s why it’s an illogical argument. An argument cannot be won by simply claiming to be an expert in the field. Besides, amplifier designers disagree on many things. I'm quite sure I can come up with another terrific amp designer who disagree with Pass.

Make two reasonable assumptions.
That there is some of legitimacy to the notion of "getting accustomed to the sound" burn in.
There is probably an element of the circuit undergoing real physical changes "during/over use" time as well.
Thus there is perceived 'burn in.'   QED....  EOD......