I have been using 7.1 directly to my amplifier for about 6 years now. I started with the M-Audio Revolution 7.1, upgraded to the Creative X-Fi Elite about 4 years ago and purchased the Asus Essence ST in November of 2009. The Xonar HDAV H6 add-on card is not yet available in the US, but I purchased one from someone that had the HDAV 1.3 Deluxe. I now have 7 channels with RCA jacks and 21 opamps that can be swapped out (3 per channel). I haven't had any problems with the drivers. However, they are releasing new drivers soon that offer automatic bit-matching and support for 88.2 and 176.4. If you do use the new drivers in bit-matched mode, you will not be able to use the master volume control. This is how it is on the X-Fi, too.
I am using Windows 7. In the MC Output Format DSP I have Channels set to 5.1, Surround mixing to No Mixing, Bitdepth to 24-bit, and Sample Rate to Source Sample Rate. This sends only the channels that are being played to the Asus drivers, but at 24-bits. In the Room Correction DSP I have set my distance levels for all speakers and subs. I have two subs, but they are the same distance from the listening position. If the Output Format was set to source number of channels, the soundcard's drivers always need to be changed to match the input channels.
In the Asus drivers, I currently have to manually change the bit rate to match my source in order to avoid resampling. The card itself only has a 24.576 mHz clock and I have read that it is best to let the card resample to 48, 96, or 192 kHz for the best quality. I'm still trying to decide which way I like best.
In the drivers I have input channels set to 5.1 and output channels set to 7.1. I also use the bass management in the drivers to set the crossover for the subwoofers. I currently have it set at 80 Hz. So far, all DVD's and Blu-rays I have watched have a 5.1 soundtrack. If I have something with a 7.1 soundtrack, I will change the input channels to 7.1. By leaving the input at 5.1, I can use the driver's Dolby Prologic IIx to matrix the rear surrounds for movies. I leave Dolby Prologic IIx off for music.
This soundcard really sounds great with excellent detail, dynamics, and soundstage. I like it that the audio section is powered directly from the PSU. I plan to keep this in my system for probably the next five years. I think that because each channel has its own I/V opamp and buffer opamps, the sound quality of the surround channels and subwoofer channel are outstanding. These were a bigger improvement over the previous cards than even the main channels.
The card does not support PAP for Blu-ray so if you are playing a a Blu-ray directly using commercial software, the lossless audio will be downsampled. However, I can rip the Blu-rays using
Another EAC3to GUI-Plus to rip the movie to an MKV with lossless multichannel FLAC audio. It takes about 45 minutes, but it can then be played in MC with full resolution lossless audio.