Pete,
My past experiences are as follows. ARC REF 3
1.Preamp usually sounds sterile and 2 dimensional
2.Highs will start to relax about 50-100 hrs.
3.Image will be more focused at approximately 150-200 hrs
4.Finally Depth and width improve dramatically at 350hr mk.
Just my .02 cents.
Keep a tuner playing through the preamp at all times.