Not sure if they break in much, as I didn’t play my pair too long before swapping out the op amps to the Sparkos (two each in the XLR input). I did notice an improvement when the op amps were replaced.
Funny, but I mentioned somewhere that the Fosi, with the Sparkos, had better resolution than pretty much any amp I have. Not by much, mind you, but I noticed some subtle things in my test tracks that I’ve listened to hundreds of times. And none of the test tracks sounded worse.
I’ve since added a pair of GaN power bricks (48volt/5 amp) and I feel that these made the background more black than with the 48 volt/5 amp power bricks that came with the Fosi. I didn’t initially hear any difference, but after breaking in the new bricks for a few days, they seemed to sound better.
Overall, though, the Fosi are quite incredible sounding amps. They are currently driving a pair of Maggie 1.7i speakers.
For reference, some of the other amps I have in home; VTL MB185; Pass Aleph 2; Cary 805B; Marsh A400S; pair of Schiit Aegirs; and a few others.
The Fosi’s do not make me want to swap in one of the other amps. I initially got these because most of my other amps generate too much heat in my small listening room during the summer months.
The Sparkos are your friend.