I'm sitting here listening to a newly constructed amp, and over the past few hours it's gone from bass heavy (about an hour and half after initial power up) to laid back and a bit light on the bass (about an hour) to a bit more forward and detailed with bass that is still a bit recessed but deeper... and it's still changing!
I remember something like this with my first ETA42; maybe it's the Elantec buffers or the Cerafine power supply caps?
It doesn't really make any sense; these are solid state components that don't get warm, there aren't any bits that really change (what, do the transistors turn off and on easier after they've had some time to distribute the grease in their gates' hinges?!), but it does seem to be happening. High-end audio is like high-frequency RF - weird and full of things that don't make any sense, I guess.
But I'd be interested to hear how other amps burn in...
Well, solid-state manufacturing isn't perfect as any other manufacturing. There might be some chemical residue in the solid-state, some mobile ions hanging around, some moisture near metal-ss contact junctions, some atomic scale obstacle in the current paths, etc. When power is applied to the SS devices, all these may stablize to better characteristics. Note that most SS devices barely experience power, not extended time, before shipping.
Besides SS devices, I am speculating some burn-in might be going on capacitors as well. But I am no expert on capacitors and can't comment.
I too found recently this fact especially with my super mini meta amp. Just like a gitar, it becomes more musical with time. I think the caps are the main cause, but the op-amps might need time to show up too.
After I swapped out the ps caps in my cd player, it sounded bass heavy and lacked dynamics. Cooked overnight and listened to the unit the next day and it sounded much better. Noticed the same thing with Corda (after cap and regulator changes): it took forever to reach the proper voltage, as was the shut down. After few days it came up much faster and discharged much faster. So I believe there definately is a break-in period for caps and ss devices.