Hello, I've waited a while to make sure of my findings... My Pass Labs 600.5 amps now have about 900 hours on them. At 650 hours the amps started to became more musical, clear, detailed, layered and 3 dimensional. It took another 100 hours more after that for the changes to be permanent. Since the amps haven't changed for 150 hours now, I would say about 750 hours for full breakin, probably due to the large power supply of each amp. Take care. Stan