I noticed even more wierd behavior: it goes GRADUALLY from normal to low-bitrate sound and back (with a period like 10 seconds) during playback in DAW! Re-choosing it as audio interface can fix it. In fact it's a lottery: when you choose it as audio interface you can get three options: 1 - normal sound, 2 - low bitrate sound, 3 - changing (described above)
The driver really needs to be fixed!!
Unfortunately I'll have to return helix next week if I don't get any response from line6 on this matter. I bought it MAINLY due to its audio interface features, but this is where it fails most ridiculously ((
UPDATE: it IS possible to change the buffer size, but ONLY trough Control Panel -> Helix Control Panel. Not from DAW. So the bug is not catastrophic, but still needs to be fixed. No bitrate bugs appeared so far with larger buffer.