I own these amps. Due to a change in speakers, I haven't run them for some time. But, IIRC, the character of the sound MIGHT change a (little) bit over the first 20 minutes or so, but nothing really dramatic. You can listen happily after 2 minutes.
As to reliability. Only one isssue. Cary loves those elegant thin toggle switches and uses them for the Bal/SE choice on the rear of the (very heavy) chassis. There is a natural tendency to lift the amp from the front, but doing so can (and, in my case, did) snap off one of these switches. Other than that - dead reliable.
I've also owned Cary 300B monos for about a decade and never had a problem there, either. Or with my Cary 303 CD player, for that matter. Generally reliable and well made stuff - IME.