I'm already watching to some DLNA NAS productNo need to apologize for using BT - I get it. its easy. (and I am an anti-BT snob for fixed installations) However, using a FLAC file probably doesn't really make a difference(per say) because sending it over BT is like using a "skinny"/"degraded" wire. But I am glad you noticed a difference ( confirmation bias?).
if you really want the benefit of using a FLAC file, put your Marantz on your network and stream from your computer - the app should be able to see your computer as long as you are on the same network/subnet.