Some of them will use uPNP to connect to "media servers" but the media server has to stream the FLAC so it down-converts as it transcodes (translates digital info to music) over the network for the receiver to play.I'm not sure where you got this information, but "down-conversion" isn't at all part of the uPNP protocol . . . the fact that it's "streaming" it from a network device doesn't mean that the audio quality is converted to typical "streaming" internet-radio quality. Plain 'ol 100Mbps ethernet has way more than the necessary bandwidth to stream 192KHz/24bit and beyond . . . so if a uPNP media renderer (i.e. the receivers that you describe) is properly designed, there is no loss of quality in transferring the file across the network.
You might take a look at the Linn DS components - they deliver pretty amazing performance, and support FLAC natively.