I use the STX with my LCD-2 at the office on a Windows 7 x64 setup. I've never had any of the screeching occur but have experienced the popping sound occasionally and it can always be pinned down to another audio source trying to be accessed while audio playback is taking place. For instance, if you go to a website where audio is playing or a video clip is embedded and programmed to run upon launching the site, and you are currently listening to the headphones in a WASAPI environment, then that audio/video from the website will try and challenge the card to output the sound. This is where the popping occurs. Also, if you have any sounds from Windows itself occur then this may cause the pop as well. That however is solved simply by going to audio and removing all sounds from Windows.
I have had the best luck with the oldest drivers available on their website, believe it or not, and they were not beta/prototype drivers. I just checked and they are still available. Version 7.12.8.1777.
All I have done is disable all Windows sounds, place settings in STX audio center at very high output 300-600ohm, 24 bit / 192, and select WASAPI output and 24 bit playback in Foobar itself. Then I make sure Windows volume slider and Foobar volume slider are both set to maximum, and only control volume through the STX audio center itself. I never go above 25-30 on the volume knob, depending on the recording, and just make sure to avoid using sites like Youtube while I'm listening to music, and I never get the popping sound anymore. If I have to access a site with audio I simply stop the playback through Foobar and start it back up again after I have closed out that tab on the browser. Also, I make sure to disconnect and reconnect the headphones each time I reboot the computer as the headphone amp in the card turning on and off at reboot will cause the popping as well.
I also made further tweaks to improve sound quality and eliminate all popping by utilizing some of the suggestions in this guide.
http://www.sweetwater.com/sweetcare/downloads/Windows_7_Optimization_Guide.pdf
Good luck.
EDIT: I don't think it matters one bit what video card / motherboard combo you are using, because like I said the popping occurs when an audio source tries to overtake the one currently playing, but I'll throw mine in just for information purposes.
ASUS P6T6 Revolution
EVGA GTX 275