Well I found my answer here:

http://www.tbrowne.demon.co.uk/comp/projects/dacs.htm

In short, the MX300 doesn't acutally have 24 bit output but it does a very high quality downsample to 48kHz via an onboard SRC.

Apparently the SBLives are supposed to have this although the quality isn't that good.

Anyway, the reason this popped up is because I noticed a drop in playback quality from WinME (24bit, downsampled to 16bit on the card), to WinXP (16bit).

Geoff