JVC TH A35 Home cinema repair, only sub and center channel working

[Deedeelabricolade]

So I have this home cinema with only the subwoofer and the central speaker working. The unit boots up with blue light, so no protection mode nor short detected. All fonctions are available (radio, cd, aux etc...).
I have the schematic of it but no idea what to look at and how. Seems that the sub and center speaker are both connected to the same relay but I don't get how the 3 other have failed as they are all powered from the same supply? How can I diagnose it safely?

[CapLeaker]

You got to start verifying that all the power supply rails are working 100%. The next component I would go after, is the STK 5channel brick amplifier. Desolder that, clean the holes and put it back in. All these style amps have a problem with cold solder joints at the pins due to heat cycling. The other thing to check is the headphone jack. it disables audio to the speakers. Did you do a factory reset?

[Deedeelabricolade]

Thank you so much for those infos, I can start digging with confidence now. As seen in the user manual, factory reset is available via scart and as this unit was used only to listen to music, I didn't check if the video was working. Btw, pretty sure I don't have any screen nor cable to check (and the remote is missing). Is that really important ?
Thank you again for your time and these golden tips !

[Deedeelabricolade]

Small update. All channels are now muted. I tested the headphone jack and I can clearly hear the output relays clicking. No sound in the headphones either. As advised, I unsoldered, cleaned and resoldered the STK. It has 30V and -30V VCC supply. All ICs seem to produce the correct voltages according to datasheet. The only thing that seems weird to me, on Q751 I have -9mV 27mV 680mV (EBC), Q706 -8mV 75mV 770mV and on Q810 1,5mV 6mV -10mV. Pretty small but I've no idea what they are for. Thanks for your time!