This model is one of my all time favorites! I have restored a few now, and enjoy doing so every time :) They all suffer from similar faults / weaknesses but one thing worth noting is don't go in all gung-ho those LED displays are very delicate (can be repaired with SMT leds but fidly) other things to watch for are connectors coming loose when jiggling things about which can cause much head scratching!
Also, be very careful with the switch panel when lifting off, it unclips from the front with three clips but do not lift up any further than is nesesary to pull the lifted clips forward beyond the front veneer. The rear of the switch panel has three clips clamping to the chassis (switch panel slides forward to release) and these can snap very easily!
You will need to to access the processor unit - and you will want to get to those pop buttons as well to clean as they can become quite badly oxidised. This results in the user pressing harder to activate the buttons which in turn puts stress on the clips holding the board to the panel, resulting in failure of the clips (usually the standby button!).
Worth stripping every last bit of this hifi (I even pull the speaker switches apart as the contacts are nearly always black with oxidation! might not make a lot of difference sound quality wise but I have had a switch fail to work, for this reason..
For the fault mentioned you could try dissconnecting the power to the amp. This will allow you to rule out fault detection. I am sure at least the standby led should remain lit though!
Will be interesting watching this thread :) Good luck!