Possibly a couple of things going on here Graham.

It seems like the person who serviced your amp changed the gain setting (this is switchable see p.21 of your manual) - hence the need to "turn it up".

But other things have happened too. It seems like the speaker protection is tripping too early. Why this should have changed I don't know.

As for the third problem ... it seems possible that there was some damage to the PCB ... perhaps from porr soldering.

I suspect the repair tech was trying to resolder some dodgy soldering and made a hash of it, did some damage to PCB tracks, and this has caused these issues.