"So, is it the tubes, the caps, or the listener?"
I would say an equal part of all three.
Why does it take so many hours to brea in arc preamps and amps?
I recently purchased a like new ARC 5 SE pre amp. The unit had less than 200 hours on it. Everything I have read states that ARC preamps take up to 600 hours to fully break in. Why is this so and what improvements can I expect to hear as the unit accrues hours?