Jens - actually, I think we have pretty much the same issue. I get no playback when forcing an implicit feedback endpoint, or when letting the Roland quirk handle it automatically. If I disable implicit feedback, it works but I get pops/clicks.

Coincidentally, I just came across this post of yours on the Line6 forums about manually tweaking the sample rate:

https://line6.com/support/topic/4426-line-6-with-linux/?tab=comments#comment...

I gave it a try today, and by forcing a sample rate of 44105 instead of 44100 I now have largely clean playback! I get the odd dropout, but it is much less frequent (on the order of once every few minutes instead of several times a second).

This is obviously an non-ideal workaround for the problem. Playback should be synchronized to the capture stream, but this doesn't seem to be getting configured correctly for our devices. Why, I'm not sure - I'm gradually gaining a better understanding of the USB audio kernel code, but I'm not there yet.

Mike