So to conclude my stereo issues, it was the amp. I ended up getting a decent set of old headphones, wrapped some wire around the rings on the Jack plug & touched it onto a couple of the wires in the multiplug as per the wiring diagrams above and I could just hear the line level sound coming though so the head unit was fine.
So things to note… even though my old amp fan ran initially and the ‘secret menu’ suggested the amp was ok on the network, it doesn’t mean the amp isn’t faulty as clearly mine was. I was hopeful it was something else but alas not.
Also, I tried another amp from another Lexus model (GS 1999-2005 I think) with the same exterior multi plug setup as mine. I was fortunate enough to find an eBay seller who was happy to send me this unit to try it. I plugged it in and it worked… sort of… but sounded awful and even messed with the radio reception. So even thought these Mark Levinson amps look the same, they’re clearly different internally. But I appreciated this sellers trust and effort in sending it to me. So I would suggest sticking with the known SC430 ML model amps - 86280 0W070 and 86280 0W250