I had a used Mk1 express that I thought was making a low hum when connected to the Amplifi. Took it back to Guitar Center and got a newer (but still returned) MkII. Same issue. But what was odd using them on GC's demo Amplifi 75 it didn't hum, but made clicking noises. Tried different CAT5 cables, and no change. The 75 was dead quiet when the pedal was disconnected. Then I ordered a NIB (new in box) Amplifi 150 and new MKII Express. So far no more weird noise issues. But I may have another issue. If I have Wah enabled on, say preset B and switch to another letter, the pedal loses the wah control and also the volume. Then if I switch back to the original letter, the pedal is still not working. All the while, the Wah LED is still lit. I expect the the wah to still be on when I go back to the original letter preset. Or this actually how it is supposed to work? (note: I can re-enable the pedal by pressing all the way forward [hard] till it clicks. That resets it to volume. Then I can set it back to Wah.) I haven't updated the software on the pedal yet. Anyone?
For more info on updating via PC vs Apple see this /http://line6.com/support/topic/10129-amplifi-windows-interface-failure/