This looks like another corporate disgrace in the making.
Denon Marantz has been sold. Your problem has made me make inquiries.
Marantz USA has turned over their entire parts business to Encompas. They are a large parts dealer. I have used them before. Recently I have found they have become virtually useless and don't stock parts.
I recently was asked to repair an expensive TV just out of warranty for friends here in MSP. Encompas who should have had what I needed did not. After a lot of searching I found an outfit that was rebuilding boards here in he US. I ordered what I needed and got core credit for the defective board, and much to my surprise the rebuilt board worked perfectly.
In your case in Australia I find your parts are handled by qualify.com.au.
They seem lousy and list no parts.
This is the contact info for Marantz parts in Australia.
Service enquiries
(03) 8542 1144
service@qualifi.com.au
Good luck with that outfit.
I did a parts search on Encompas and they do not have the part you need here in the US
I think existing laws need enforcing. It seems firms are flouting the law and their obligations with abandon.
I dread having to replace one of my pre/pros. I have become more and more convinced over time it will have to be a DIY design and build. More and more are doing this now.
I think these forces really are threatening this hobby. My guess it is heading to extremely expensive boutique units, DIY or a sound bar.
I have become increasingly concerned about this issue of late. Unfortunately I don't think your problem is now out of the ordinary and set to become the rule.