Since you mentioned line out, I'll assume this is a G100-II and attach that schematic. If this is wrong, please post appropriate schematic.
SS can be difficult to trace because of direct coupling and feedback paths. If all the DC voltages are not correct, it will throw everything off and you will not be able to trace signal. So proper DC voltages at all transistors is first priority.
Unfortunately, this schematic doesn't tell you much for DC voltages.
Transistors can breakdown under load or at idle. So a transistor that checks ok with diode check can still breakdown under voltage in circuit.
Signal collapse is due to output transistor not supplying current as you mentioned, but it can be other things than the drivers. The output transistors themselves or their emitter resistors, or even bad solder could be problems.
It sounds like you have good signal without load, but check at the speaker out rather than line out, with no load.
If it is good unloaded, then yes the problem is in the final stages.
Measure DC volts idling at TR309,310,311,312 first with no load, then with load connected.
If you are using a limiter lamp, note whether bright or dim.