"Can't get transistors" is just silly.
The line outs are from the preamp. So it would be the problem so far. Now the LAST thing I am is a replace all the caps all the time guy, but in an amp this age (over 45 years) all those little 1uf caps in the signal path are instantly suspect. But before changing them all, the schematic has DC voltages all over it, so go through the preamp and compare yours.
I would also apply a steady strong input to the amp and follow it through teh preamp to see where it goes away.