I buy Matrix combo defect.They was not enough loud and sometimes they have strange voices.
The problem was big ceramic resistor.They have broken legs near the PCB board.If I will not try to move, I will never find.Maybe you should try to move elements on board, if they moving, they are broken or bad solder.I found this problem on many SS amps.Somethimes they broken.
Take time to look carefully.Sorry, but German is not my good side.Maybe you have complete diferent problems, other hands maybe will this help to somebody in future.
Zoran