First make sure all your drivers are up to date. Keep running Line6 Monkey. You'll need to reboot then run the Monkey over and over until no more updates are shown. If that doesn't fix it read on.

I had the same problem on my MBP with my UX2. When I ran System Profiler (Finder->AppleMenu->AboutThisMac/->Click on "More Info") it showed the USB port as being USB 1.0 speed (12 mb/sec), not the high speed USB 2.0 speed (480 mb/sec) the UX2 needs.

Line 6 says not to use an external USB hub, but that is what fixed the horrible cracking and popping and other noises for me.  I put an external powered USB 2.0 hub between the UX2 and the Mac's USB USB's input. The sound immediately cleared up, I even have other stuff on the 2.0 hub (an iLok key, external keyboard and track ball, iPhone dock...). None of that seems to interfere at all. The sound is now excellent.

Maybe Line6's drivers and hardware are not kicking the MBP's USB ports into USB 2.0 high speed mode. The Mac seems to think these Line6 devices are USB 1.0 instead of the USB 2.0. So it reads and writes the USB port at too slow a speed and the signal breaks up.

I hope this works for you. Good luck Don!

- Laurie

I have a GuitarPort. Just bought a new computer andI have spent the last week trying everything to nail down this crackling problem. Sometimes it worked, sometimes it would crackle no matter what I did. I COULD NOT get any consistant reaction. Sometimes I could play all night and it would be fine, most times I couldn't hear over the crackling. Installed a powered USB buss port ....... instant fix!