I sympathise with your challenge. I struggled with one of my Quad 405s because I prefer the 'one component at a time' replacement policy when faultfinding, but the practice failed me with the 405. What David says in post 6 is so true where directly coupled audio amps are concerned - a single component failure tends to lead to a cascade of faults. Then when one faulty transistor is identified and replaced, you find that the amplifier has transformed itself into an evil transistor destroyer.
The only solution is to root out all the faulty semiconductors before powering up again.
An approach which I've found useful is, with power off, to test transistors individually in circuit simply using the 'diode test' facility on a digital multimeter to establish that each one still contains two junctions and no shorts or open circuits. A silicon junction will normally read between 0.6V and 0.8V on diode test in its forward biased direction. In the other direction, it may read anything from a few Kohms to infinity depending on the surrounding circuitry.
Incidentally, Murphy's law ensured that I had to go through this twice with my faulty 405: having got it working again very nicely, one careless slip of a test prod, a flash bang, and I had a whole new fault to find!
By the way, I too can recommend Dada as a fast and reliable supplier of genuine parts for the 405.
Martin