Start-up current of amp triggers circuit breaker
A B16 Circuit Breaker = Leitungsschutzschalter tolerates big insurge currents, f.e. 4 times 16A = 64A for 1 second. Maybe this circuit breaker is very old or had been damaged by a massive short in the past. A swap of that CB would be inexpensive, below 5 Euro for a new B16. 

The most easiest idea would be to insert one or more power line extensions between the socket and the amplifier. That would increase the power line resistance.

If the electric installation is made for it,  a C16 or even D16 would tolerate higher insurge currents, but is more expensive.

Another possibility is to add one or more insurge current NTCs in the amplifier's power wiring to decrease the high current, if it is an old amplifier without any. A newer amplifier's NTC(s) could be damaged.

They are normally black coin-shaped elements, see the photo taken from 
https://www.ametherm.com/ptc-thermistors-vs-ntc-thermistors-for-inrush-current


It is also possible that the amplifier has a special power switch with a resistor switched in series. When the switching operation starts this resistor will be bridged after a few miliseconds.  But in that case there should be a hint in the manual that this unit should only be switched by the built-in power switch and not by a switch of an extension or a wall switch.