Different reviewers give good reviews for different reasons unrelated to the actual performance. It doesn't matter what your speakers are, either, especially if you do the comparison on the same system. You can use any speaker and listen to music and the audio will be fine on either the onboard chip, or a sound card. I was very interested in the concept of changing out an op-amp and changing the dynamic of the board's audio. However, in practice it is not as capable as the same concept on a separate audio card (the Xonar in particular has a more effective approach with eight upgradable op-amps). In particular, when it comes to noise the ZxR is far superior with a mere 124dB SNR. To be fair, the onboard SNR matches the Z, and Zx.
However, when you take a complex video game like AH and compare it on both devices the sound card will be superior (given that the onboard chip and card both use the Core 3D chip). The reason is simple as outlined by Skuzzy. To prove it all you have to do is test it, as I have. So, you can stop calling it 'theory.'