So, first things first, trying to switch mains to the amplifier.

As said, I didn't want to modify the amp itself (too much) and wanted the power button to still function. At first I checked the service manual (which contains the complete schematic + PCB layout!) to see if I could rewire the power switch and connect mains downstream of the switch.

In the excerpts from the manual below, the power button is marked red, refdes 1201.

It directly switches L, but there is a bridge (refdes 9204, blue arrow) in the schematic that is not present in my version. When placed, this bridge 9204 bypasses the switch 1201 when placed. I guess that's meant for an always on version of the amplifier that can only be put in standby. (Note: When in standby, relay 5202 just downstream of the power switch stays disabled, cutting power to the amplifier stage.)

These bridges are just a kind of (through hole) jumper wires that can be used to bypass part of the circuit in other version. Perfect for putting my own wire in.

For the neutral, it wasn't clear to me where I could solder to. There were several points, but I didn´t really know what they looked like. So I opened it up and it seemed that refdes 1223 (blue arrow) was a header post, to which a wire could easily be soldered. So my bypass function shouldn't be a problem!

While I was disassembling the amp though, I removed the power link (refdes 216, not in pictures below) from the power switch 1201. As the switch itself is almost at the back of the unit, there's a long plastic link, that physically connects the power button on the front with the switch. While removing it though, I broke the power switch 1201, meaning it wouldn't latch on anymore.

That sucked, because it rendered my amp useless, unless somebody was keeping the switch pressed. So, as a temporary work around, I soldered my bypass wires to a generic JST B2P3-VH, which is also used for mains input connector 1221. That meant I could wire the mains input cord either to connector 1221 on the PCB or to my bypass connector. Now, plugging the other side of the mains cord in an Shelly Plug S, I could switch my amp on and off!

Problem one fixed! Ish.

A more permanent fix has yet to follow. Also more on how the power link 216 connects to the power switch 1201 and what I did to bring back power button functionality..