Ok, I've managed to get decent drivers for my motherboard-embedded CMI8738 card. I've installed M-Audio Delta DIO2448 WDM drivers, which use same hardware, and I can say that, in case of a single stream, and setting the wave mixer slider at max., the digital output is bit-perfect, no matter if the stream is 44.1 KHz, 48 KHz, directsound or waveout. It is pretty clear that in this case kmixer doesn't alter the signal at all.
I have the same onBoard sound. Does this produce bit-perfect results only with these special drivers?