I've worked in electronics for decades and am confident in most gear, but this is not a job I would willingly take on without a schematic and layout and design will vary between brands and models so anything done will be case specific.
I'd look to find the input to each amplifier channel internally, as that would be the easiest place to take signal from and would be the natural pre out point anyway. If you're lucky, that will be a cable or cables that connect the line level section and power amp boards where you can break out the signal.
Best of luck. Considering how easily an amateur can damage something like this, I'd suggest just buying an AVR with pre outs.
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