Another update:
I managed to insert a EMO EN-70HD in the ADSL link upstream of the router on the theory that galvanic isolation AHEAD of the router might be desirable to decrapify the incoming signal before it even travels in the Ethernet link downstream instead of directly before the switch.. Shazam! Significant increase in detail retrieval and soundstage. Never ceases to amaze how much fine tuning is needed on a proper digital link.