Hi,
I have had the same problem with an Amplifi 150: 4 blinking LED and no LED around the main button. No sound produced by any way(direct plug guitar, bluetooth, USB).
Strange, all the other fuctions are working. Bluetooth OK, USB OK (Amplifi seen by the PC), firmware upgrade working. I have opened my amplifier.
All pover supply voltage OK. By the way, it is possible to convert from 220V to 115V by adding a jumper (I've moved from Europe to the US: no more transformer !!!)
I had to buy a main board and replace the old one. Amplifi is running again! (cost: $125, not happy)
I have noticed a Micro SD memory card. When removed, nothing is working. I have removed this Micro SD from my NEW BOARD, made a COPY to the Micro SD
on the OLD board and reinstalled back to the OLD board. Then upgraded software. Guess what: the original (OLD) main PC board is working again on the Amplifi.
The problem is only a software problem. Hint: I have made a copy of the new Micro SD for the future.
Yours, André