OK, I suspected that the bitrate was high even for hi-res lossless stereo music files, hence the reason for asking the file types you are actually using.
Ironically, it looks like the built-in Windows 7 UPnP/DLNA media server is transcoding in push mode, rather than pull mode, so opposite to my original thoughts (which were based on my guess of you using a lossless music file format, rather than something like AAC)! Good news is that L16 is the standard stereo 16 bit depth / 44.1kHz sample rate LPCM audio signal format, which is what PCM streamers produce from lossy music file formats like AAC anyway.
So there's no loss in sound quality with the Windows 7 push to mechanism transcoding the AAC files to L16, when streaming to your Pioneer receiver. Windows has picked up on the AVR's capabilities of supporting L16 over DLNA, but somehow not AAC too, hence the reason for its media server transcoding AAC to L16.
L16 with its resolution of 16 bit /44.1kHz also matches your observed bit rate of around 1400 kbps (stereo 16bit/44.1kHz actually equates to 1411.2 kbps to be precise).
Only mystery is why your around 250 - 300 kbps AAC files which are not being transcoded in pull mode, are being observed with slightly higher bit rates of around 300-400 kbps. Since the AAC files aren't being transcoded, their bit rates should be unchanged. Are you still saying you are seeing the slightly higher rates in pull mode?
Last edited by Cebolla; 06-15-2014 at 03:01 PM.