your problem hiss wise is either an op-amp, or a pair of transistors being an amplifier.
But while you are inside, check the output voltages. you should get 5,12,-5,-12, those sorts of values. or very close(within 0.1 volts)Also, re-seat any socketed chips. Matrix's have real issues with that.
Always have your the volume on your synths as high as they can go. Gives your mixer less to do, and your recordings will come out better.
The only thing you can do to extend the life of your gear is to not hit them with hammers or operate them submerged in seawater.
They break when they want to. They always will.
If you got more questions post away.