Essence ST uses Burr Brown PCM1792A which is a high end stereo-DAC, your Yamaha-receiver most likely uses a much lesser model and a multichannel one to that. However, the bottleneck is not the DAC but most likely the receiver itself, to fully appreciate the difference in favor of Xonar ST I guess you need better gear. You can use the card as a HP-amplifier to get better sound than from the receiver, and as such the card is marketed, as a headamp-card!
It's not entirely impossible the difference may shine through despite the receiver, I have a Sony-receiver in the livingroom using a Wolfson-DAC and a Sony bluray-player also using a Wolfson-DAC but a slightly better model. Not as good as PCM1792A, but it can easily be heard that the analog out from the bluray-player sounds better than the sound through HDMI(on stereo-Music that is!). For one, one is bypassing the DSP of the receiver entirely, and I think it would be the same in your case if your receiver has a "source direct" button. You can't bypass the DSP of the receiver using HDMI.
even if you run HDMI to the receiver and doing Pure Direct, although you're bypassing all the DSP, you're still using the receiver's DAC and Amp. you're essentially just running the digital to digital output from your STX to the receiver, but your receiver is still the one doing the digital to analogue output to your speakers. but i would say, and willing to guess that running HDMI to your receiver and letting your receiver do the work will 'probably' yield better results than running analogue RCA to your receiver, in which case you're decoding the signal and amping the signal twice, which possibly will degrade the signal by a whole lot. when possible, you should avoid amping twice. but who knows, unless if your receiver has really ****ty DAC, which i doubt it.
Edited by howzz1854 - 5/3/13 at 11:01am