Many interesting points have been raised in the last page or two, and if you just use all those suggestions you should be able to get the Hiface 2 to work on Win 8.1 with USB 3.0 ports and with the latest drivers--and sound a little better.
First, the firmware, I just updated my Hiface with the firmware one of the engineers at M2Tech sent me, and it indeed solves all my problems since I changed computers (which I kind of knew were USB 3.0 related), no more pops or crackling every 30s! So I encourage all of you to update your firmwares. Note you need to reinstall the drivers afterwards.
Next, even though it's not absolutely necessary, I got the latest 2.23 drivers off the Luckit website, changed the usual strings of text in the two files, installed with driver signature disabled, and the drivers work fine. There's a new control panel since 2.19, and the "always on" option is not longer there. I saw on some Chinese forum that there's a way to enable this "always on" option forever using an old control panel exe separately but I haven't gotten that part to work yet (the only issue is that I might get some stutter when I first start playing sound).
And I would still suggest getting all the latency settings as low as possible in the control panel, and getting / making an external battery power supply for the Hiface, as I've suggested in the past...
Anyway, I don't know what influenced what, but I do feel a marked improvement in sound after updating drivers from 2.15 to 2.23 and changing firmwares. I had tried 2.19 a few weeks ago and felt a slight improvement, which I attributed to placebo, and I don't believe much has changed between 2.19 and 2.23, so I think much of the change is because of the firmware (whether it's because it's inherently better or because it plays more nicely with USB 3.0 I don't know), or at least half drivers, half firmware.
I'd love to upload the latest modded drivers and firmware here, but I'm too lazy to find out how and where I can host them, so if someone wants to make the files available, just PM and I can email them to you.
Edit: Chinese forum page if anyone is interested; one dude has a link for an "always on" solution lower on the page. I downloaded it but couldn't get it to work, since it's made from drivers 2.16 files (old panel) as a solution when using drivers 2.19 (new panel) and for another XMOS based device (iFi something). Good luck figuring this Schiit out...
http://www.pchifi.cn/forum.php?mod=viewthread&tid=103661