Some more observations:
I've measured the latency on the same machine running Windows 7 using Centrance's latency test utility (http://centrance.com/downloads/ltu/) and got much better results than under Linux. That's a bit disappointing...
I don't know if these latencies come xrun-free since I don't know how to detect xruns on Windows. On the other hand, telling Jack not to complain about xruns from Alsa (softmode) I can get down to 64 frames/period and 2 periods/buffer without any noticable glitches or crackles (playing a sine test tone). Maybe the Windows driver just doesn't complain either?
Nevertheless, I'm a bit surprised that just starting Jack without any clients will result in a flood of xruns at small buffer sizes. Is there anything we can do about it? Is this to be expected? Is this a flaw in Alsa, Jack or the USB stack? Is there a way to debug these xruns?
Any suggestions?
Regards, Felix