So, I tried Yamaha, who sent me an updated RS232 pdf, but unfortunatly the update did not have anything to do with power settings.
However, I solved it today by accident. Turns out, that the Ack-command wasn’t working for some reason. I don’t know why it wasn’t working – it was set as ack command in V-Control, but when I used a showmessage(ioresult) in the Ack Command it suddenly started working.
I rewrote the Ack-Command to give me the required feedback. My current amp driver (which basically only contains Power On/Off) is attached to this post, in case anyone is interested.