I've got racks full of solid state power amps that have seen all forms of "duty": everything from left on most of the time to being switched off and on once a day. Two of them date from the 70's - others from the 80's and 90's. I've never noticed a correlation between "mostly on" and "off and on" with respect to failure rates. In fact only one amp has ever failed. But after a $100 repair it's been back in service ever since. One other amp had two lights in each power meter that, one by one, eventually failed. When the last one went, I took the amp in to the shop to replace all four. At the time the tech identified a handful of parts (caps, resistors and transistors) than were out of tolerance. Again after a $100 bill everything was back up to spec. But you know I never did hear a difference in that 30 year old beast - and it's still powering my centre channel speakers (where I'd be most likely to notice.)

So as to turning 'em off and on or leaving them powered? I now turn mine off and on simply because eight stereo power amps tend to consume a bit too much power, even at idle, to be left on all of the time. Except for outright major failure, with the occasional bit of maintenance every decade or two, I expect my solid state amps to potentially outlive me.

And that leads me back to the original post for this thread, and the very cogent argument put forth by SH. If my amps actually "broke in" during their first few hours, or days, or even weeks, after being first turned on, then that process surely must have continued until, by now, decades later, they'd be producing nothing but pathetic bleeps and chirps. But since that's not the case, my conclusion is that break in is a myth in the case of such solid state devices.

I'd also consider that my speakers broke themselves in the first time they travelled a cycle or two. And my phono cartridge? Once around a groove would have been about it. After that they're all very slowly wearing out - not breaking in! But luckily for me I can't hear a difference. (Then again I've got audiograms of my hearing response going back to 1981 and it certainly has changed over the years. So even if I were to postulate that my speakers sound different now than when I got them in 1979, given the fairly significant change in my hearing, I doubt that any change attributable to the speakers over their lifetime would come within an order of magnitude within what's happened to my hearing.)

Jeff Mackwood