[LAU] Latency and USB interfaces

Hi all, Sorry for this uneducated post, but I am sure there are many people here who know a lot more about this than me. This discussion about the Scarlett 18i8 has got me thinking about latency with USB devices. Is there an accepted buffer size that USB audio interfaces should be able to use with jackd now? I do seem to be getting xruns with 128/2 (even occasionally with 256/2 on program startup/shutdown), but I have not made any particular effort to try to tune my system for low-latency use. At 64/2, the sound breaks up in a kind of electronic sounding way (like a robot voice from the 70s), and linuxsampler (for example) complains the buffer size is too small.. Not quite sure what's going on there. I had a vague idea from discussions years back that there was a hard lower limit to usb latency, and that all devices, regardless of quality/chipset would share the same properties in terms of latency under USB. Presumably this is not the case any more (if it ever was??) Also, how meaningful is the reported jackd buffer size in terms of actual latency? When I was trying to help with fixing the kernel bug on my 2i4, I was told that the actual time between i/o packets was actually much lower than what was reported by jackd. Could this really be true? James