I don't have as much experience with those players. I believe Winamp can be configured to use WASAPI, but it's not the default. Also, many features are only available in its paid version but are free in the players I mentioned.
Bitstreaming is just the term used to say that the bits coming from the audio soundtrack are being sent to the receiver unchanged in any way. It can be used to describe the audio being sent over HDMI or S/PDIF connections. (e.g. DTS 5.1 and DD 5.1 can be bitstreamed over optical. If they are down-muxed to stereo PCM, though, then they're not being bitstreamed.) That term isn't actually used by any of the free media players, although it is used in the menus of some of the commercial DVD and BD player software packages. They also can play files from a computer's internal disk drives.
FWIW, I found that when I configured the Windows Audio Playback device to provide the maximum number of channels provided by my receiver (7.1), then when I played a stereo audio file, all of the audio players that I tried would use only the front two channels and leave the others channels present but silent -- preventing Dolby ProLogic from being applied, for example. I discovered that only a few players could be configured to automatically send a 2 channel signal for a stereo audio file and use more only when playing a multichannel audio file. Only those which included support for WASAPI would do that, and they had to be explicitly configured to do that. It wasn't the default. This is separate from bitstreaming, though, since there are many audio file formats which receivers can't decode. They have to be translated by the player software into something the receivers do understand. (I'm using HDMI from Windows 7, which is different from what you're using.)
Selden, how does your above description address the problem of the OP?