Ok so you've established the right channel is running an offset.
As MArk suggests it may not be rail voltage 30-50v but an intermediate
voltage 20v you are measuring.(20v rails would mean a very low power amp!)
check this by measuring the voltage on the centre pins of the o/p tr'sw. one
will be +40v perhap, and the other -40v
If this is the case then yes the outputs are probably ok.
Also you can test the o/p tr's (unplugged this time) by looking for a short
across any 2 of the 3 pins. you will read some conductance across certain
junctions but we are most specifically looking for a short.
Assuming these are ok and that the rails are 40v plus then it really is just
a case of testing all other small transistors in that channel (6 or 7
nearby) looking for a short or (more difficult to identify) an open
junction.( not measuring like a diode - conducts one way, not the other).
I would not expect to find just a faulty cap or resistor without a blown
piece of silicon.
If you're going to solve this you need to learn about transistors and how to
measure them.
personally I always use an analogue meter/ on a low resistance range, and
one gets a feel for the readings (junctions on B-E B-C) (no junctions C-E)
(no reverse leaks) (PNP or NPN types)
A digital meter takes a long time to settle readings but does usually
have a diode range that can be useful.
It's here how us time served engineers earn our money- identifying
efficiently the dud bits and not replacing more than is really necessary.

Hope that helps for now.
Report back with your findings

AW