>If some R2R DACs offer filters that artificially, but functionally improve separation, soundstage etc without being a detriment in most music then I wouldn't mind trying that out.

For the above criteria, take a look at the DAC's from Denafrips, like the Ares II, now known as Enyo. I don't have personal experiences with their products, but when I was researching and considering them, these are the general impressions that were consistently conveyed - with the caveat that many report more pronounced results with speakers, rather than headphones.