HIFI amplifier: diagnosing power amplifier stage

I have a Sony HCD-MD373 HIFI which experiences odd issues:

there is no sound on attached HIFI speakers
there is sound on headphones plugged into the headphone jack
there is sound on headphones attached to the HIFI speaker connections
Signal path (see top right hand corner):

The above descibed phenomena lets me conclude:

the speakerspeaker output relay is working correctly
mute control is working correctly
To me this leaves the following options:

amplifier is defective
overload detection is defective
"DBFB DET" is defective (I believe that is some kind of bass feedback loop)
Amplifier board circuit:

The amplifier fuses in the above board are ok.

Questions:

Can anybody take an educated guess what the issue is likely to be?
Any hints on how to proceed diagnosing this with only a multimeter?
Can the Sanyo STK4132MK2 power amplifier IC be defective given that it is still able to drive headphones?
Can the power supply to the power amplifier IC be defective given that the amplifier still drives headphones?