Hello, I don't know how the repair went in the end, but I bought this amplifier today.
1. I would try a firmware upgrade, a usbA - usbA cable is enough for that.
2. I saw somewhere on the web that you can buy replacement boards from the amplifier for normal prices, or maybe the manufacturer will know about it.
3. It's a shame that service manuals are currently unavailable, the EU could make it mandatory to publish them.​