Based on what i read here at head-fi, the ST and STX were the same thing except the ST was more expensive and theoretically had better fidelity if you used optical SPDIF output. since my motherboard is short on PCI slots and i use coax instead of optical, i got the STX. BIG MISTAKE. my previously very stable windows 7 64bit system (3+ weeks uptime, not one BSOD in the 9 months since install) now BSOD's almost hourly. it often happens when going from silence to any sound playing (music in foobar, video, game or even just a system beep). i looked around on asus's site and apparently a lot of people are having BSOD problems with this card - but the drivers are ancient (relative to typical sound card drivers) and there isn't a peep from anyone at asus about this issue. a little more googling and i found threads like this http://my.opera.com/rejzor/blog/vista-x64-radeon-hd5850-xonar-essence-stx-bsods i tried that fix (uninstall ATI HDMI Audio) and I still get BSOD's. I've contacted Asus and they said the card is not defective so even though i bought it brand new just a couple weeks ago, it's basically trash. The card is 100% fine according to their diagnostic tool but the drivers are simply incompatible with my $700 video card. major problem, zero support. i probably would not have had any issues had i bought a $200 sound card from any other company. what a mistake it was to buy an asus product.