Well, already I've discovered probably the most compelling benefit to using the ASUS Xonar sound card as my DAC.
I had been running my Velodyne Impact Mini subwoofer from the Icon-1's line-out jack, which fed it a full-range signal. As I commented in another thread, this configuration doesn't provide any type of high-pass filtering to the S-1's tiny 3" drivers, so with high-level bass signals, the drivers could be sent into a paroxysm of overexcursion, in spite of the subwoofer's presence. This had an extremely detrimental effect on transparency at high volume levels, as the cones thrashed themselves to death and distortion soared.
The Xonar sound card, like most current multichannel sound cards in its price range, features an adjustable crossover on its subwoofer output, with the selectable ability to high-pass the main channel outputs. So I'm currently running the Icon-1 from the card's main L/R outputs via a 2.5m AudioQuest Mini-A cable with a shielded jacket and solid conductors, and the Velodyne Impact Mini subwoofer gets fed from the sub/center output of the card. Experimentally, I settled on a 95 Hz crossover frequency.
This configuration has unlocked an incredible new reserve of power and clarity from the system at high power levels (enough to make the Razer/THX Mako system green with envy!) simply from relieving the S-1's tiny drivers of any deep bass content. Where the cones had throbbed visibly before, now they barely budge, and only when watched very closely at almost painfully loud levels. The transparency the system had lacked before at high volumes is restored.
Of course, none of this would work if the ASUS Xonar card wasn't such an incredible device in its own right, with a measured 117dB SNR at its main L/R outputs, and superb DACs and opamps on board. The sound card is encased in a metal shield to prevent the infiltration of EMI from other sources in the PC, and the card has plenty of filtering capacitance on-board. Rest assured, it's an extremely quiet source. It is at least as quiet as the Icon-1's onboard DAC.
(Addendum: Setting your PC's volume control to a low setting can bring the noise floor up and prevent the card's entire dynamic range from being utilized. My gain structure is thus: I run the Windows Mixer volume control at 30%, and the Icon-1's volume control at 12 o'clock. The ASUS Xonar D2 is run in 5.1-channel output mode to enable the subwoofer channel. Tweak the subwoofer levels and crossover to taste.)
