I probably should have mentioned something about those bias transistors, they are mounted in a really flimsy way on these amps. I unscrewed mine to stop myself bumping them while working, but other Yamaha amps like the M-series use screw-hole packages (like TO-126) with thicker legs and have a much more durable mounting setup.
I didn't want to suggest something I hadn't tested, but since we're here now...
I wanted to fix the problem permanently, so I poked around until I found a close match for specs (to the 2SD-400) in a TO-126 package, and came up with the BD-135.
I'll post some pics of what I did later if you're interested. I have mounted everything, but am not able to test yet.
If you want to fix the amp you will definitely be able to.
@avionic helped me fix mine, I can share what I learned.
You can use these for the main output devices:
https://www.digikey.com.au/product-detail/en/sanken/2SA1494/2SA1494-ND/3929367
https://www.digikey.com.au/product-detail/en/sanken/2SC3858/2SC3858-ND/3661814
That will be the biggest single-part expense (i.e. not the end of the world) and everything else is cheap, the only real cost is time.
With the help of the DBT and a little creativity with meter measurements (with the amp unpowered but components still in circuit) I've managed to fix a few now without removing many parts. Obviously in-circuit measurements are not always accurate, but having the good channel to compare with helps give clues about problem parts.
The first thing I'd do is remove the 2 main output transistors on both channels from circuit (circled ones in my pic). You can just desolder and move legs off the board. Then solder a 470Ω 2W resistor across their B-E pads on the board. At this point the amp will live on the DBT + 100W until you get a relay click again. New outputs won't go in until that happens.
You'll need to find some subs for the driver transistors too, the subs I used are now discontinued (Toshiba 2SA1837 and 2SC4793).
Edit - bdent still has these, it wouldn't be crazy to buy a few, as they're no longer produced:
http://www.bdent.com/2sa1837-toshiba-audio-power-transistor.html
http://www.bdent.com/2sc4793-toshiba-audio-power-transistor.html
They also stock the aforementioned outputs:
http://www.bdent.com/2sa1494-sanken-audio-power-transistor.html
http://www.bdent.com/2sc3858-sanken-audio-power-transistor.html