On the subject of bit-depth, my new DAC turned up today, and I noticed something interesting:
Media Center is outputting 32-bit via ASIO (and I guess the driver truncates it to 24-bit) so when I start playback, or when tracks change, the 24-bit indicator lights up - but if I have all DSP plugins disabled, the indicator drops down to 16-bit after about half a second of playback with 16-bit lossless files. So that's something else which confirms that Media Center is doing absolutely nothing other than adding zeros when outputting a 24-bit signal with 16-bit files. (not that I doubted it for a second)