After using my Guitar Port for some time now here's the settings that work for me. I don't have any problems with these settings!

Go to System> Preferences> Sound Preferences and make sure everything is on the Line6 (OSS) and on Default Mixer Select the ALSA version. This Config seems to work for me. Make sure your "Monitor Track" is muted due to some feedback when using Jack.

When recording with Jack only use "Capture_1" for your recordings. "Capture_2" is used by the Line6 software as the "DI" for the tuner and for some reason it has alot of noise.

As for the ALSA support I'm clueless! It works under ALSA in Jack, so I don't know about that!

My only problem is to find a nice amp emulator program for Linux.