I would be interesting if someone bought a new pair of monoblock amps and burned in one amp and then compared the two. I bet the sound difference would be very audible between the two amps.I had a pair of Benchmark AHB2 monoblocks that were used everyday for a year. I got another brand new AHB2 and could not tell a difference from the older pair. However, I do remember reading the designer of the amp stating the AHB2 does not need any break-in.
I bought a lot of gear in the last 12 months (over 10 items, integrated amps, DACs, preamps, amps) and all of them except the AHB2 required break-in.